For Tony, it's something that has always been there, in the back of his mind nestled between the numbers and equations. Hell, most of the thoughts occur in forms of equations. Calculations of height and force of impact or angles and the pressure behind a blade or even how much energy needed to stop a heart. They're always there, some ordinary, some innovated and unique but always there.
The thoughts never leave but that doesn't mean he is always thinking about it, his mind doesn't work like that. Sometimes the equations of death get pushed forward more then the rest and he actively finds himself contemplating them. He doesn't go through with them though(in fact, he only tried once) because Tony is not suicidal. He isn't, the thoughts are just there, always. He knows that isn't normal but then again, nothing about Tony has ever been normal.
He hears the Avengers joke about it, hell, he jokes about it but mainly because by joking he can deflect any true suspicion and distract them from how he does have those thoughts. It takes a while for him to realize, though, that the team never mentions it around Steve. Oh, they joke and such but never about Tony's...tendencies and he soon finds himself wondering why. Had Steve warned them off as being inappropriate? He can only imagine how put off Steve would be of his issues, not that it is an issue. Tony doesn't want to die, he really doesn't but the thoughts and numbers are there, that's all. He is just aware of the possibilities.
It doesn't even occur to Tony that maybe Steve just doesn't know, can't see the danger that lurks in the back of Tony's mind. At least, it doesn't occur to him until a series of incidents makes Steve suspicious and Tony accidentally says something he shouldn't have.
The first happens after a battle with the Enchantress and her ax-guard. Her magic is screwing with Tony's suit, knocking certain features offline or just causing general glitches. He's fine with that though, has fought battles with intense damage before and won and that was working alone. Here he has backup, teammates who can and will watch his back. And that goes both ways.
So when Tony sees that Steve is about to take a hit to his unprotected back from an overly large ax, calculations running impossibly fast in front of his eyes, he doesn't even hesitate. His boots scream as he pours everything into them in order to get there in time. Then there is a jarring impact that seems to rattle his very bones and the HUD flashes red with warnings. The suit ruptures, seals are broken, circuits fry and then the HUD dies. Thrown into darkness with Steve's yell of his name ringing in his ears, Tony struggles to catch his breath. His chest hurts so badly that he knows without looking the reactor is damaged. Still working if the faint blue glow is anything to go by but definitely broken.
Tony lays in the dark, just breathing through the pain, for he doesn't know how long. His mind is oddly blank, numbers still scrolling through behind his eyes but they mean nothing. Finally, after what feels like forever but is probably only minutes, Steve's voice sounds outside the armor, begging Tony to open the helmet. If Steve is asking that then the battle must be over so Tony chokes out the override code and is relieved when the suit responds, face plate snapping up.
Immediately, Steve's worried face fills Tony's vision and he forces a grin, “Hey, Cap.”
“Tony, thank god. Are you alright?”
“Yeah, I will be once I fix a few things.”
Steve's worry doesn't seem to ease, “fix what? Do you need to go to the hospital? You should go to the hospital.”
“No, no, it's fine, I promise. A hospital can't fix what's wrong with me but I'm gonna need a lift back to the mansion.”
Frowning, Steve nods sharply. “I think Thor can carry you back.”
Thor, appearing on cue, leans into Tony's field of view. “Aye, Man of Iron, I can transport you back in no time!”
“Thanks, Thor.” Tony can't suppress his grin and honestly doesn't try too hard. Thor is amusing and fun, certainly his favorite on the team(not counting Cap, Steve is just on a whole other level).
“I'll catch up with you.” Steve says as Thor hauls the suit and Tony upright, which hurts, no lie there, but Tony hides it well.
“You don't need to worry, Steve, I'm fine. Do what you need to do here.”
Despite Tony's assurance, Steve remains serious. “I'll be there.” he just says again, sounding more like a promise then anything else.
After a finality like that, Thor lifts off and true to his word, he has Tony back at the lab in the mansion in no time flat. Which is good because the damaged reactor is getting worse.
Thor is hesitant to leave Tony alone but he's not keen on getting out of the armor and working on the arc reactor with the God right there. It's just a thing Tony has. So he manages to talk Thor into going back to the fight scene by mentioning any possible lingering magic left behind and with a reminder that Steve is on his way. Not that he actually expects Steve to show but Thor doesn't know that.
Finally, with Thor out of the way, Tony calls JARVIS online and the A.I. along with Dummy helps him get the suit off. By the time the metal is shed, Tony is almost exhausted and he collapses into a wheelie chair with a pained sigh
Dummy, unusually efficient Dummy, sets a new reactor down on the desk beside him and Tony smiles, patting the robot clumsily on the head. “Good boy.”
Dummy whirs and clicks, likely pleased by the praise and Tony leans back in the chair to look down at the reactor still in his chest.
Sure enough, there is an ugly crack straight through it in a diagonal arch that suggests the impact of the ax was the cause for the damage. That man's weapon, despite looking archaic, must have been insanely powerful and made of nothing from Earth. Not much had the strength to crack the arc reactor, usually only vibranium could even hope to damage it.
“Well, this is gonna hurt,” he mutters to himself, reaching for the spare.
“Sir.” JARVIS' voice stops him, “Captain Rogers is requesting entrance.”
Tony actually freezes for a moment, thoroughly thrown off his guard. “Hold him off until I've switched out.”
“Very well, sir.”
Steve actually came. The thought circles dizzily around Tony's mind and his hands shake minutely before he gives a mental shrug and pushes it off. Hand to his chest, Tony steels his resolve, grips the reactor, twists and pulls. With a click and static buzz, the reactor comes free and Tony grits his teeth against the sudden spike of agony. He lets the damaged reactor drop from nerveless fingers and fumbles for the spare. It's clumsy going, getting the replacement in and Tony has to try several times to get the connectors back in place. Finally though the reactor clicks solidly where it belongs and Tony lets out a huge gust of held breath.
The pain begins to recede but Tony just knows he'll be sore for a few days after all this. He hears the subtle hiss of the lab door sliding open and Tony sits up straighter, bending down carefully to pick the spent reactor off the floor. He probably shouldn't have dropped it in the first place, it still had some juice left after all and could blow up the entire block the mansion sits on.
He sets it on the table just as Steve appears and Tony grins at him from the chair. “Hey, Steve.”
“Tony.” The Super Soldier's eyes take in the genius' appearance and the cracked reactor. “Are you okay?”
“I'm fine, Steve.” Tony says, amused and a bit exasperated. “I told you that already.”
“Well you don't look fine.”
“And you are stubborn. I'm okay. Tired and sore, sure but I'll live.”
Steve nods but he doesn't relax and his eyes keep snagging on the broken arc reactor. “Why did you do that?” he asks before Tony can think of someway to distract him.
“Jump in front of the attack like that.”
“Oh.” Tony blinks, taken aback by the question and not actually sure why he's asking. “Because there was no way you'd make it out of that.”
“So, what, you thought you'd have better odds of surviving?” There is something unusual in the narrowing of Steve's eyes but Tony doesn't know what it could be so he elects to ignore it.
“Well, not really. I don't usually think of things like that anyway. I just kinda thought, 'hey, look, Cap's about to die, really can't let that happen'.” Tony shrugs but Steve's look has changed to something like alarm.
“What do you mean you don't think of things like that? I'm pretty sure thinking whether or not you're going to survive an attack should be a high priority.”
“Maybe. It's not really anything I've actively concerned myself with to be honest.” Feeling a bit steadier, Tony slides off the chair to grab the reactor and puts it someplace safer until he gets the chance to dispose of it properly. Which mostly means he pulls open a drawer and shoves it inside before dragging an undershirt out of another drawer. He's just about to tug it on when Steve's hand on his wrist stops him. The blond looks bewildered.
“I...Tony, what does that even mean?”
It's at that moment that he realizes Steve truly doesn't understand and that means he doesn't actually know. Which leaves a thick, uncomfortable feeling at the pit of Tony's stomach and he knows he has to deflect because if Steve doesn't know then Tony certainly doesn't want him to.
“Um, I...it was just a joke, Steve. I don't know why I did that, okay? I didn't even think, I saw you in trouble and acted, that's all there is to it.”
But Steve looks uncertain, like he doesn't quite believe him and knows he's missing something but just isn't sure what. He doesn't say anything but lets Tony pull free and tug the shirt on, eyes still studying him closely. Tony tries to ignore the staring, patting down his shirt and thoroughly hiding the glowing reactor behind dark cloth.
Numbers are spinning through his mind distractingly and he welcomes the disruption, pulling them together behind his eyes in ways that make sense for calculations over his armor. It needs repaired and now that he's fixed he can start on that. Tony turns away to gather pieces of the suit together and Steve sighs behind him.
“Okay. Just...be more careful, please. I like you alive.” And with those parting words the sliding doors of the lab swish shut as Steve takes his leave.
'I like you alive.' Tony mouths to himself, chest feeling tight as an effect of something other then the arc reactor. Pushing aside the simple numbers of painful death by screwdriver, Tony focuses everything on his armor and doesn't leave the lab until a day and a half later, when everything is shiny and perfect.
After that first close call, Iron Man continues to take harder hits then the rest of the team, usually as a result of sweeping in to take an attack they couldn't avoid but nothing as severe as the first. Still, it means Tony is often bruised more then the others, though his clothes or the armor hides it easily. He really doesn't mind, either, he likes that he can be there to help. He doesn't regret it but he is aware of how it must look and Tony knows it's only a matter of time before Steve decides to confront him again. Tony's really not looking forward to that conversation but can't see a way to avoid it. He won't stop what he's doing on the battle field, it works and they all walk away alive at the end of the day so he won't apologize either.
And as suspected, the dreaded confrontation finally occurs after a long battle with a swarm of Doom-Bots. Tony knows it's coming because Steve had refused to look or speak to him once on the way back to the mansion and he retreats to his lab to at least shed the armor before it happens. He's not sure what's going to happen or what this is going to do to the friendship that's been growing between them but he does know one thing. He likes Steve, he likes Steve a lot and if it ever comes down to himself or Steve, he'll always pick Steve. That's just how it is.
Just as Tony is setting aside the last of the armor, big pieces held up by supports and the smaller scattered across his work tops, Steve comes into the lab like a blond whirlwind of righteous anger. Tony can't even get his mouth open before Steve is shouting.
“Are you suicidal? Because I'd like to think there is some reasonable explanation for why you are so stupidly reckless in battle when I know for a fact you're not actually stupid!”
And, wow, he got to the heart of the matter a lot faster then Tony was honestly expecting.
“Well?” Steve demands, standing within arms reach of him and glaring expectantly when Tony doesn't respond right away. “Do you have a death wish or what?”
Unfortunately, anticipating this moment has not prepared Tony for it at all and he can't think of a worthy, distracting come back quick enough. At his continued silence and what Tony can only imagine his face looks like, Steve's hardened expression falters.
“Tony?” he breathes, worried and bewildered and Tony suddenly has the horrible realization that Steve had meant those questions rhetorically and had only wanted him to explain himself. So, crap, his own lack of reaction just screwed him over.
“No!” he blurts finally, avoiding the Super Soldier's eyes, “I don't have a death wish. I mean, it's not like I go around wishing someone would off me or anything. Seriously, if I had that attitude I really wouldn't have lasted this long.” Which is true or else he never would have made it home after his kidnapping.
“Not suicidal either.” Tony says, cutting that trembling tone off. “Well, not really. I mean, I don't want to kill myself. It's not like that. It's just the way my mind works, you know? It thinks things in these numbers and stuff and I can't really ignore them but it's not like I do anything about it.”
Steve's eyes are wide, he's pale and he looks like he ought to sit down but Tony doesn't dare suggest it.
“I'm a genius, Steve, you remember that right? And you said it yourself, I'm hardly stupid enough to be truly suicidal. It's just, I'm wired a bit weird okay? I do calculations for everything, most of them independently of my own stream of conscious, and some of them just so happen to include the best ways I could die with anything from an electrical outlet to a socket wrench. I think them, then they get pushed away and I continue on life as normal.” Tony's not sure how much of that Steve understood because he doesn't look relieved.
“You can't, ever, do...do that. Ever.” Steve sounds as unsteady as he looks and Tony is seriously getting worried that he's somehow broken the man.
“I won't! I promise, Steve. Listen to me, I don't want to die. I don't. If I did it would have happened years ago but I don't. I don't.” Taking a dare, Tony reaches out, resting a hand gently on Steve's arm, willing him to understand.
“Okay. Okay, good.” Steve takes a huge breath, letting it out slowly but doesn't relax much even if he doesn't brush off Tony's touch. “But then why are you so reckless?”
“Reckless? I'm not reckless, Steve, trust me. I do the calculations and every time I interfere it's only because the alternatives are less acceptable.”
“Acceptable.” Steve repeats, frustrated. “What does that even mean?!”
Tony shakes his head and steps back, realizing that he's going to have be frank and already knowing that Steve won't like what he has to say. “It means, Steve, that if there is an incident where I can prevent life threatening damage to you or anyone else on the team, I will do whatever it takes to do that. If, in the course of preventing, I get hurt as a result that outcome is acceptable to the alternative of me not interfering.”
Steve stares blankly at Tony, jaw a bit slack before he closes his eyes.
And, see, this is why Tony didn't want him to know. Steve cares far too much for anyone and everyone and no life is expendable the way Tony is willing to trade his.
“You compete idiot.”
Tony blinks, leaning back against the work top while waiting for more. That can't be all Cap has to say.
“If you really believe that then you are nowhere near as smart as you and the rest of the world think. You are not sacrificial, Tony.” Steve opens his eyes to pin the engineer with a steely gaze, earnest and determined all at once. “You are one of the most important aspects of this team and more then that you are an amazing person. I realize that you don't see that so I want you to listen to me and believe me on this. No one will ever thank you for getting yourself killed for their sake. Especially not me. You don't have the right to determine how much your life is worth because you're biased. Understand me?”
When Tony didn't answer, too busy staring in surprised dismay, Steve narrows his eyes and starts walking closer to him, only stopping when there was barely a breath of space between them.
“That's a yes or no question, Tony.”
“I, um...yes?” Tony breathes, practically able to feel the natural heat coming off Steve.
“Are you asking me or telling me?” Steve says with a tiny smirk as he plants a hand on the work top by Tony's left hip.
He thinks Steve is joking just now which is such emotional whiplash Tony has no idea what to do next. “Telling. Yeah, definitely telling you. That's not to say that I agree exactly but I hear you and really at this point that's ump!”
Steve, rolling his eyes just a moment before, cuts off Tony's ramble with his mouth, pressing it gently to the genius' and stealing the words from him.
“Oh.” Tony gasps weakly, fingers curling in the Captain's shirt. “I was not expecting that.”
“Like I said, idiot.” Steve says with a grin, slipping his other hand beneath Tony's top, causing a thrill to shiver down the other's spine.
“Yeah, well, I never said I was perfect.”
“And I never said I cared if you were.”
Tony smiles and for once the equations rolling in the back of his mind are practically invisible for all the attention he pays them. After a moment of soaking in the touch, Tony pulls himself together enough to be serious again. “I can't promise you that the jumping in front of you stuff will stop, it's kind of automatic, honestly.”
“That's okay, I just want you to realize anything that ends in your death is no more 'acceptable' as something that ends in mine. Got it?”
Tony hums thoughtfully before nodding, the least he can do is try. As long as Steve understands it isn't something that would happen right away then Tony can work with that. Besides, with Steve so close, pushing aside those annoying equations of how best to strangle oneself with hydraulic hosing is pretty easy.
“So, is there anything else you wanted to say or are we taking this to one of our bedrooms?” Tony asks with a cheeky grin, slipping his hands down Steve's impressive chest to hook his fingers into his belt loops.
Steve flushes slightly and gives Tony that 'aww shucks' smile that never fails to make women(and Tony) melt. “My room,” he purrs and grabs both of Tony's hands away from his belt to start dragging him away.
So yeah, it's safe to say that for the next few hours Tony didn't imagine a single death related thought.