"What the hell, Dean?" Sam hissed.
"I don't know."
"No, seriously, what the hell."
"I don't know," Dean repeated.
Sam shot a glance over Dean's shoulder at the guy who had seen them for what they were - not the FBI agents in professional suits here to investigate what appeared to be a repeated crime, but a pair of hunters looking to hide something supernatural from police eyes. The guy seemed pretty harmless in his expensive-looking coat and suit; whoever he was, he wasn't a cop. Or another hunter.
So what Sam didn't get was how the guy figured them out.
"Who is he?" he asked, nodding at the guy. "Not a hunter, clearly."
Dean rolled his eyes. "Yeah, no kidding." When Sam kept looking at him, expecting a useful answer of some kind, Dean shrugged. "I don't know, maybe he read Chuck's books."
"And what, he thinks they're real? Dean, the super fans that were at that convention didn't believe us, why do you think a guy the police let into a crime scene would?"
Dean frowned at Sam. "Then what's your explanation, huh? Cause if you've got a better idea, I'm all ears."
And right when Sam was about to say something a little snarky in return, a hand slammed down on his shoulder, and he flinched. "Agents!" the guy said happily. "How are we doing?"
Dean grabbed the guy's hand and forcibly removed it from his shoulder. In the guy's defense, he only winced a little bit; impressive, given how strong Dean's grip got when he was in a bad mood. Sam shrugged off the other hand and gave the guy a look.
"Okay," he said weakly. "I'm gonna guess that's a "not very well"."
"Who are you?" Dean asked lowly. "How do you know about hunters?"
"I wouldn't say I really know much about hunters," he said carefully. "It's mostly just you." Sam and Dean glanced at each other, then down at the guy. He blinked at them. "You are Dean and Sam Winchester, right?"
"How do you know those names?" Sam asked, trying to use his supposedly intimidating height in his favor. It didn't seem to work, though; the guy just grinned.
"I'm right, you are them! Oh man, this is so cool!" He tried to look over Sam's shoulder by stretching and standing on his tiptoes; when that failed, he ducked down under Sam's arm. "So what is it this time? Vampires? A werewolf? Or ooh, ooh, maybe a demon?"
Sam shoved his arm down, blocking the guy's view of their very interesting, very problematic crime scene. Dean stared at the guy.
"Seriously, who are you?"
"Castle!" the detective they'd taken the case from - Detective Beckett, Sam thought - called out, walking over to them. The guy flinched, turning to face her with an innocent smile... which probably made him Castle, then. "What's going on?" she whispered. "Ryan and Esposito are talking about getting this case back." She turned to look at Sam. "Is that true, agents?"
"Uh..." Sam glanced and Dean quickly, then turned an uncertain smile toward Beckett. "Possibly?"
One of her eyebrows twitched. "Really."
"Yeah," Dean said, nodding confidently. "On closer inspection, it doesn't look like your case is related to ours after all. Similar MO, but enough subtle differences to make it clear it's the wrong guy. We'll hand it back over to you, on one condition."
The eyebrow twitch turned into a full-on quirk, raised just enough to show both interest and irritation. For a second, Sam was kind of jealous of how expressive that eyebrow was. "And what's that?" she asked.
Dean swung an arm over Castle's shoulder and tugged him in close. "Let us borrow this guy for a couple hours? He's expressed some very interesting insights, and we'd like him to take a look at our case, see what he comes up with."
Beckett glanced suspiciously between Dean, Sam, and Castle. Sam tried a winning smile, which didn't seem to impress. The expectant look on Dean's didn't either. The slowly growing, terrified look on Castle's, on the other hand - the one that said he was beginning to realize how far in over his head he was - that made her smirk.
"Alright, agents," she said, smiling evilly at Castle. "Give me back my case, I'll let you have our consultant." She held out a hand to shake. "Deal?"
Sam shook. "Done." Deal made, Beckett walked over to her uniformed officers, reacquainting herself with a case that she'd thought was out of her hands. Meanwhile, Sam and Dean strongly suggested to Castle (by way of their hands pushing on his shoulders) that he follow them out of the roofless building.
"But - hey!" Castle protested, trying to turn around. "I didn't even get a chance to look at the body! How am I supposed to help you?"
"Oh, trust me," Dean said, shoving the so-called consultant out the door. "That is not why we grabbed you."
Castle's eyes lit up. "So it's about your hunt?" he asked in a low voice. "Tell me the truth - is it a demon?"
Sam sighed. "It's not about a hunt."
"We don't even have a hunt!" Dean added angrily. "What we've got is a civilian who knows too much, poking around in things too big for him."
"But - but -" Castle waved a hand in the direction of the police tape. "The dead guy - the mysterious burst of light? The vanishing John Constantine? That's not a hunt?"
Sam paused mid-step. "John Constan-" That was just - well. Actually... Dean, having also stopped, tilted his head, thinking about it. After a minute, he nodded, and glanced sideways to catch his brother's eye.
"Huh. You know, I can kind of see it. The whole trench coat, messy suit thing?" He gestured in a way that would look ridiculous to most people, but to Sam was an approximation of the length of Cas's coat and the way his tie was always twisted around and loose.
"Yeah, I guess so," Sam agreed absently. Now he was wondering if Jimmy had ever read Hellblazer.
"Wait, wait," Castle said slowly. "You know the guy?"
Sam looked at Dean. Dean looked at Sam.
Then they took notice of their location - cops all around, rubberneckers and busybodies alike peering at the scene, news crews hefting cameras and microphones. Castle looked between the two of them, the crowd, and back.
"Maybe we should have this talk somewhere else," he suggested. "I have an apartment in SoHo? It's quiet... we won't be interrupted any time soon?"
After a moment's hesitation, the police consultant was shoved into the backseat of the Impala, and they were headed for Manhattan.