If you asked Crowley, he would say it was all Aziraphale's fault for letting the angels on to the trick of (technically) permissible possession.
Aziraphale, of course, asserted otherwise - after all, demons had been possessing vessels for thousands of years before the Apocawasn't  - but in sober reality, if he hadn't shown up at Tadfield wearing the body of fifty-seven-year-old psychic hooker Madame Tracy, the Heavenly Host could never have figured out how to contain their spirits within mortal forms, Zachariah and Azazel could never have collectively developed their Holy/Infernal Scheme, and the whole nasty mess could have been avoided. It really could have ended when Adam Young said "no more messin' about" and banished every supernatural entity  from the Earthly plane of existence. The Apocalypse Mark II would never have gotten started, and little Sammy Winchester would have grown up in an unburnt suburban household with two miserably divorced parents just like your average demon-free middle-class kid.
If you asked Dean, he'd say it was all Pastor Jim's fault for teaching Sam how to pray.
Sam, of course, would argue otherwise, but again, facts are facts: if Sam hadn't decided to turn his face up to heaven and pray for strength on one long cold despair-filled night after Jo and Ellen Harvelle had been blown to kingdom come along with several hundred hellhounds, none of the rest of it would have happened. Sam would have simply spent the rest of the week drinking large amounts of alcohol and pretending very hard that neither he nor his brother were crying, and then gotten up next Thursday to keep on fighting the good fight and get possessed by Lucifer and try to kill Dean and trap himself with Michael and Lucifer down in the Pit for all eternity, or at least until he came back again a few days later without a soul.
But that was in another time. In this time, in this place, Zachariah noticed Aziraphale. Sam prayed. And the rest, thanks to the writings of the prophet Chuck , is history.
 Angels had to stop possessing people after the coming of Christ, because Christ (taking, presumably, a similar view to Adam on the subject of possession) had firmly restricted all the angels to Their Own Bodies And No One Else's, Thank You, Unless You Asked Nicely For Permission First. Until Tadfield, Aziraphale had been the only angel who'd figured out the trick of Asking Nicely.
Demons, of course, followed the opposite rules: you could possess pretty much anyone you wanted to as long as you didn't ask nicely and they didn't say yes. This is why demons are so much more likely to possess small creepy children, sensible old ladies, priests, hookers, and sheep . Although most of them will deny the sheep thing.
 Thanks to an overactive imagination and a healthy diet of horror novels and Anathema's magazines, Adam Young did not actually consider ghosts, tunnel-digging Tibetans, pagan gods, wendigos, werewolves, vampires, skinwalkers, and other assorted horrific entities to be supernatural, and therefore did not banish them from Earth along with the angels and demons, to the everlasting regret of the Brothers Winchester and their fellow disgruntled (and frequently dismembered) hunters.
 And, some authorities argue, the apocryphal works of Holy Fanscribe Becky Rosen, although others reject the validity of these texts due to the author's nondivine origins and the text's excessive preoccupation with Sam's abs.
* * * * *
"...thy will be done, on Earth as it is... as it is...as it..."
Sam couldn't say the Lord's prayer anymore.
He hadn't been able to for a while now. Every time he tried, his head buzzed and his lips cracked and bled and he had to break off and finish the thing in pieces. It was this, more than anything, that convinced him that no matter what Dean might think, no matter how long he might have gone without Ruby's blood inside him, he was no longer 100% human. But Sam had always felt that it wasn't the form of the prayer but the effort involved that was really important, and at least, he consoled himself in his darker moments, God had left him human enough to try.
Even if he was pretty much convinced by this point that God was no longer listening.
"...in Heave- in Heav- Heave- on Earth as it is- Fuck." Sam rolled his forehead against the rough plaster wall, and tried to keep count of the minutes in his head. How long would it be until Dean came up to check on him? Dean had retired to the hood of the Impala with a bottle of Jack and, seemingly, no intention of moving, but Dean usually checked up on Sam at least once an hour nowadays, and it was nearly 10PM. Dean wouldn't be happy if he came back to find Sam praying; he'd never had much patience for piety even as a kid, and these days he had none at all.
Truth be told, these days, Sam didn't either. The last time Sam had prayed with any kind of sincerity was long before Dean died, before Dad died, back in the days when all the Winchesters had to worry about was the implosion of their own private family universe. But he thought Jo and Ellen deserved something more dignified than mere ash-smudged photographs and the drunken stupor Dean and Bobby were currently trying to drink themselves into, and praying for their souls felt like the right kind of gesture.
"Heave- Heav- He-"
It wasn't working. Sam heaved out a sigh, wiped at the blood dripping down his nose, and decided to move on. He could finish the rest of the liturgy after he'd said his piece. A broken-up prayer felt weirdly more appropriate, anyway - broken like Jo's back, like Ellen's hopes, like the shell of an old tavern burned to the ground.
"God rest Jo. God rest Ellen. Save their souls, and watch over them, and God- God give me the strength to carry on. God give me enough power to keep my brot- to keep everyone safe, God, please, give me strength to keep anyone else from dying, to save them, please, God, let me save them, let me save Dean-"
-And his body flew back and crashed into the dresser with a sickening thud, limbs flailing out stiff and sharp like he'd just been electrified, whiteness blanking out his vision as his brain suddenly streamed with light, and light, and light-
He opened his eyes to find his face mashed up against the set of ugly neon coupon clippings spilled out across Bobby's guest bedroom. He couldn't remember how he got there. His vision - until he blinked, and the words swam sickeningly into focus - was blurry, but he didn't hurt anywhere, which pretty much eliminated the possibility of either a concussion or a hangover. So why the fuck was he lying on the floor?
"Hmm," drawled a rich, plummy British voice in Sam's ear, as his eyes blinked at the flyer lying inches from his nose. "50% off Pacific Northwest Getaway Spa. Promising. And I'm certain Canadians would have better taste than to use that peculiarly garish shade of off-pink in their advertisements. My dear man, could you possibly tell me if I'm in the United States of America?"
And then Sam doubled up and vomited all over the hideous coupon-strewn carpet, because the strange plummy British voice was coming from his own mouth.
"Get out," Sam choked, through the bile filling his throat, trying to remember where they'd left the salt and the holy water- out in the trunk and away in the car with Dean, oh fuck. (And why did Dean have to pick now of all times to drink himself unconscious - Sam couldn't damn well exorcise himself and Bobby couldn't climb stairs and by the time Dean came back it would be too late and oh God Dean-) "Get the fuck out, you fucking-" He choked on the vomit and gasped again, trying to scream, and felt another slow wash of horror creep down his spine when he realized his lips were already moving without him.
"Language," said his own throat sternly, if hoarsely, coolly spitting out the rest of the vomit. "Oh, dear, you humans do make such horrible messes." With sick helplessness, Sam felt his arms moving of their own accord, pulling him awkwardly up the overturned dresser. He strained hopelessly, trying to move what he knew should be his muscles, but it was no use - it was like trying to walk through a psychic brick wall. The demon hadn't left him nearly this lucid the last time he'd been possessed, and it was so much worse now, a thousand times worse because he could feel exactly what it was doing to him, what it was making him do - oh shit, he would be able to feel everything it was going to make him do-
It didn't feel like a demon, though. It felt different. Brighter. Burning and clean and shining, nothing like the cold dark slime of a demon. In fact, it felt almost like...
Oh no. Oh hell no, Lucifer couldn't have.
"I didn't say yes," Sam rasped, hauling control of his throat back from the- the light-thing with a sheer wave of rage. "I didn't say yes, you bastard, you can't-"
"Ah." Fuck, did Lucifer sound embarrassed? "Yes. Well. I'm afraid my standards of consent have perforce become a little more - er, flexible, these days. Um. Extenuating circumstances, and all that. But you did pray for divine intervention, you know, so even if this isn't quite what you hoped for, you can at least be reassured that your prayers are being heard and answered. Isn't that nice? Now if you'll just let me know - quickly, please, and with no more fluids - whether or not I'm in the United States of America, I would be very much obliged."
Sam's limbs were abruptly released, windmilling him forward into a fall that was broken by an abrupt tug, not physical but somehow psychic, hauling him up by the shoulderblades. Over his shoulder, clear-edged with the same kind of dangerous unreality that had surrounded his visions of Jess, he caught a lightning-quick flash of white feathers.
...Wait a minute.
It occurred to Sam, belatedly, that he was probably possessed by an angel. Not an archangel, not Lucifer... no, he'd gone and Jimmy Novak'ed himself a real run-of-the-mill ordinary angel from Heaven.
"I- um- I'm in South Dakota," he said, and was stopped short by the intensely weird feeling of alien joy blooming somewhere at the back of his head.
"Finally. I'm terribly sorry, my dear man, but it seems I'm going to require your services for rather longer than anticipated. There's Somebody in your country that I quite urgently need to find."
* * * * *
Dean was not having a good day. It had started with a hangover the size of Lake Michigan, and proceeded with the unpleasant realizations that a) he was still being stalked by the Archangel Michael, b) Sam was still being stalked by the ex-Archangel Lucifer, c) he had apparently passed out last night on Bobby's kitchen floor and his clothes smelled like puke, and d) Jo and Ellen Harvelle were still dead, bringing the grand total of Living People Dean Winchester Gives a Shit About down to two.
It said something truly depressing about his life that this was not actually the worst morning he'd had that week.
However, it became the worst morning of the week - in fact the worst morning of a lot of weeks - the instant Sam walked into the room, because Sam wasn't moving right. Dean knew his brother's body in many ways better than he knew his own, knew all the tiny tics and habits of motion that made up Sam, and Sam wasn't moving right.
It took him less than two and a half seconds to get up off the floor with the Colt out and pressed to Sam's forehead.
"What are you?" Dean snarled.
"Uh," said Sam.
This was not, precisely, what Sam had been expecting. He had, for example, not expected Dean to be quite so quick on the uptake, or to be sleeping with the Colt (a disturbing new development that couldn't bode well for Dean's mental health), or to have gotten so drunk he passed out on Bobby's kitchen floor in his own vomit.
Dean was not in any condition to be having this discussion. Dean wasn't, frankly, in any condition for anything but a long six month vacation, possibly followed by early retirement. Sam kept very carefully still, and wondered quietly how exactly over the past year his big brother had gotten so helpless.
"I am an angel of the Lord," Aziraphale said, and the Colt twitched and Sam went shit and shut his own mouth very firmly and started trying to convey harmless harmless harmless without making any sudden moves.
Dean's face shuttered, quicksilver agony flashing across it so quickly Sam wasn't sure if he'd imagined the expression or not, before falling, inexplicably, back into boredom. He stuck the Colt back in his belt, turned away and ambled over to the sink, and started washing off the messy side of his face, groaning loud and obnoxious like the two of them were having some kind of normal conversation.
"If you feathery dickwads won't leave my subconscious alone, you think you could save the dream-visions for sometime my head's not splitting open? Jesus, Michael, that was the first time I've slept in three days."
Sam hadn't known Michael was visiting Dean's dreams, although given the frequency with which his own Lucifer-hallucinations stopped by these days Sam probably should have suspected something similar was happening to Dean. Lucifer usually dressed up as Nick or Jess or sometimes Mom whenever he came creeping, and it had been horrible - absolutely horrible, one of the worst sights of Sam's life (which, given his life, was saying a lot) - to see something so evil wrapped up in Jess's skin, but Sam felt suddenly and painfully grateful for the forms the Devil had chosen. He didn't know what he'd do if Lucifer had ever turned up looking like Dean.
No wonder Dean was so tense around him these days. Sam felt a fist of sudden fear twist in his gut as he wondered what kinds of things Michael was saying. If it was anything about what a goddamn idiot Dean was for sticking by Sam so long, it couldn't be anything less than the truth.
While Sam was busy reeling, Aziraphale nabbed control of Sam's mouth again.
"Michael? Er- really? Dear me. Mr. Winchester, I can assure you very emphatically that I am not Michael, although I am most definitely of divine origin. Samuel and I have finally managed to come to a little agreement, and I am going to be cohabiting your brother's body for the foreseeable future. Consensually, of course." He stuck his hands in Sam's pockets, and beamed up at Dean with the sort of little-boy smile Sam knew Dean couldn't have seen on his face in years.
Dean's face crumpled.
"Oh shit," said Sam, trying to move forward, at almost exactly the same time Aziraphale tried to say "Dear me". The result came out something like "Dgqroutt-" and sounded like a baby ghoul dying.
"Sam," Dean said, in this awful tiny little broken voice, and staggered to his feet, nearly falling. Aziraphale opened their mouth again and Sam very firmly wrenched it shut.
"Sam, you- I- How could you? I said no, I turned down Michael, I fucking said no- how could you say yes to Lucifer, after- after you and Cas and- Bobby's legs- and Ellen and Jo, and what the fuck good is any of it anyway when- I- you- fuck you."
"Dean," said Sam in horror, "are you crying?"
"You can't have him." Dean choked out, through - oh God, they were actual tears, Dean was crying - and threw himself bodily at Sam, sending them both crashing to the floor.
"Dean, you idiot!" Sam hollered, relentlessly quashing the angelic urge to defensively smite things. "Get off!"
Dean didn't waste any time messing around; he slammed all full two hundred pounds of his weight down on Sam, pinning both hands up above his head before Sam realized what was happening, and he glared down at Sam as if looking at a stranger. Something in Sam's hindbrain went "oh" and he felt himself go still all over as he realized: stupid girly tears aside, Dean looked dangerous.
"I don't know what you're here for," Dean said, and if the tiny broken voice had been scary, this new flat expressionlessness was fucking terrifying. "But I don't give a damn what you're planning to do with me. I'm not saying yes to Michael, and I'm not killing Sam."
"For God's sake, Dean-"
"I know he's in there somewhere," Dean continued, a hint of expression creeping into his voice for the first time. "Sammy? You hear me? I don't care what kind of shit Lucifer made you agree to. It's not you, okay? It's not your fault, I forgive you. I-I will forgive you, later. When we get you out. I'm going to get you out."
"I am Sam, you fucking idiot-"
"We can sort out-" and Dean gave a pained little hitch of breath, soft, through his nose- "You can sort out whatever Lucifer might've made you do to Bobby later, after- just- You keep fighting him, okay? Just keep fighting, Sammy, you've got to get yourself free-"
"Fuck it- Dean- I'M NOT LUCIFER!"
Dean blinked, and sat back abruptly on Sam's stomach.
Dean's eyes narrowed. "Christo."
Sam rolled his eyes and pulled out the wings. 
He waited a beat or two for Dean to stop gawping at him, did the little mental handwave that made them go away again, and wriggled one wrist pointedly. "Do you mind getting off me, please?"
Dean's eyes went wide, and he scrambled up and away. A crashing noise came from the other room, and Bobby rolled belatedly around the corner wearing nothing but a filthy terrycloth bathrobe, a sawed-off, and an enormous scowl. Dean was apparently not the only one who had woken up hungover.
"What the Sam Hill is goin' on in here?"
"Sammy's possessed," Dean said, and Sam started silently cursing his brother six ways to Sunday as he found himself once again facing the business end of a gun barrel.
Oh dear me, said Aziraphale. I must admit, I didn't expect the introductions to go quite like this.
Shut up, featherbrain, said Sam.
Out loud, he said, very carefully, "Bobby, I am not possessed by a demon."
"Christo." Sam kept his eyes carefully wide, and waited another beat until the shotgun barrel was lowered.
"See? Told you. You can go get the holy water if you want to, you can do all the tests you like-"
"Although I would very much prefer a warning before you try to count any nipples. Last time was rather embarrassing for everyone involved."
And there, right on cue, was Aziraphale, with the world's most angelic timing. Sam yelped as the gun swung up again, accompanied this time by a faceful of holy water.
"Bobby, he's not a demon! He's an idiot angel who can't keep his- awrgk-" (and apparently Aziraphale had had enough, because he surged up over Sam's mental shrieks) "Hello. My name is Aziraphale, and I am an Angel of the Lord. I am currently residing in Samuel's body. I am not Michael, I am not Lucifer, I am certainly not a demon, and I very much wish I could say I was pleased to meet you and the other Mr. Winchester, but I'm afraid that lying is a mortal sin." Sam felt his face stare back and forth between Dean and Bobby dubiously, before Aziraphale grudgingly added, "Although I assume you both improve considerably on further acquaintance, judging by all this squishy pink affection Sam keeps oozing around in here."
Dean and Bobby blinked. Bobby's eyes narrowed.
"Is this some kind of joke?"
Sam rolled his eyes, and flung the wings out again.
"These look like a joke to you?"
Dean's eyes went wide again with very gratifying awe, and Bobby goggled for a bit.
"But- I- Sam-"
Aziraphale pursed Sam's lips and tutted, which was a truly bizarre sensation.
"Oh for goodness' sake. Even Madame Tracy's young man wasn't this much trouble, and he attempted to exorcise me with his thumbs!"
"Hang on. We were talking to Sam," Dean said, eyes going dark and suspicious. Bobby was still busy staring at the wings. "How can we talk to Sam if you're possessing him?"
Sam felt his own eyebrow rise, which felt only slightly less weird than the tutting. He had never been able to lift only one eyebrow before. "Did you expect me to just shove his soul out or lock it away in his head somewhere? That would be terribly impolite. It is Sam's body, after all; I am merely a guest."
Dean glared. "Yeah, right, just a 'guest' who can take over his free will whenever you fucking well feel like it, you self-righteous douchebag!"
"I can take control back whenever I want, okay? Honest. And listen, Dean-"
"I cannot believe you would agree to something like this," Dean breathed, glaring. "After all we know about those feathered assholes. Sam, what in Hell's name made you think this would ever be a good idea? Of all the stupid-"
"Dean! Would you shut up and listen to me!"
Dean glared, but he also shut up, something Sam had only ever seen happen before around Castiel.
"Lucifer can't possess me now," Sam continued. "He can't even find me. As long as Aziraphale stays with me, the Apocalypse can't happen. We're safe, Dean, do you get that? We're safe."
"And that makes it okay for him to wear you around like a meatsuit? Well, fuck, I guess as long as we're safe, that makes everything better, I might as well go give Michael a call right now-"
Sam flinched, but before he could get his mouth open to yell, Bobby cut in.
"Let's deal with the angels we've already got first, okay?" He turned to Sam, wheeling the chair around to face him, pointedly ignoring Dean. "You say you've really still got your free will, son?"
"Yes sir." Sam kept his eyes firmly on Bobby's.
"And it's really you in there?"
"Last year, you got a mall pedicure off a free coupon, and told me that it changed your life." He turned to Dean, trying to project calm. "The first time I tried to ride a motorcycle, I was ten years old in a town called Gopher, Indiana, and we lied to Dad and told him I sprained my wrist sparring. You hate flying more than anything except grape-flavored jolly ranchers. Two nights before your deal came due, one year ago, we-"
"All right!" Dean interrupted loudly. "All right." He took a deep breath. "Fuck. I don't believe this. If it's so easy for angels to let their vessels live, what about Jimmy, huh? If this Aziraphale really is an angel, why hasn't he taken you over?"
"For the last time, Aziraphale is not like Castiel-"
"Settin' aside the whole control thing," said Bobby, in his loudest 'idjit' voice, "what in hell made you pick Sam? All right, so Lucifer can't possess him now, but from everything I've heard you feathered monkeys are right there on board with the Apocalypse. Can't be too safe for you, taking over the most wanted vessel in Creation."
"I was not aware of Samuel's role in my brethren's plans until he informed me of it," said Aziraphale, sounding annoyed. "I was, actually, not aware that my brethren were making plans until a few hours ago, and I came down to Earth intending to address the issue. Sam was simply the first suitable vessel I could find within continental America. I assure you, I am enjoying this situation no more than any of you. I would hardly have chosen this body - he's far too ridiculously large, and he wears nothing but disgustingly ill-fitted flannel, and his hair-"
"Oh my God," said Dean, in a tone of horrified revelation. "Sam. You've been possessed by a gay angel. A gay British angel."
"Technically," said Aziraphale, in an even prissier voice, "angels have neither gender nor nationality-"
"Where the fuck did you even find a gay British angel?"
"I think you're both kind of missing the point here," said Sam.
"The Antichrist is being possessed by a gay British angel-"
"Sam's not the Anticrist," said Aziraphale.
Three sets of eyes blinked in surprise.
"...What?" said Dean.
 Three if you counted trenchcoat-wearing angels.
 Sam was actually kind of proud of the wings. They were gratifyingly white and mystical-ish, and Aziraphale seemed to have figured out how to keep his essence under control enough for the feathers to show through all dramatically backlit by heavenly light without burning anybody's eyes to cinders.
Also, he was pretty sure they were bigger than Castiel's.