Notes:

I implied that Cas is human here, but I also wanted it ambiguous if you wanted Angel Cas. So yeah. He's hunting with them, at any rate.

As always, cross posted on ao3 under sonofabitch_awesome if you'd like it downloadable. :P

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Four Times Cas and Dean Didn't Fool Sam (And One Time He Was Totally Done With Their Act)

Sam wasn't quite sure when exactly Castiel and Dean first got together, but he knew beyond any doubt that they were. They were both in better moods, even during hunts – hell, when one of them was injured, they still managed to sound hopeful as they finished off whatever creature they were taking down. Castiel was matter of fact in proclaiming the monster's future demise, while Dean was always joking and upbeat even if he had bones broken through skin and tasting the air.

They were also way too at ease with each other these days. Sometimes on stakeouts in the Impala, Cas sat up front with Sam and Dean, and Dean didn't say a word. Not a single syllable about personal space.

The eye thing had gotten worse, too. Sam found himself repeating the same information an extra two or three times before Dean clued in and ripped his gaze away from Castiel's eyes.

And oh yeah, then there was the time he caught them kissing.

Sam had been looking up some information on his computer, tracking the beginnings of a case, when he'd realized he was out of beer and innocently figured he'd just go snag another one.

Dean had been taking a while in the kitchen, and Sam should have known. He'd almost yelled for Dean to bring him out another bottle, but thought better of it after realizing he had to get up anyway to hit the bathroom.

He'd taken one step into the kitchen and thought Nope, don't really need another beer after all yet at the sight greeting him. Dean and Cas's mouths were vacuum-sealed, Cas sitting on top of the preparing table with his legs locked around Dean's waist and Dean's hands tangled hopelessly in Cas's hair. Gross.

As Sam spun and retraced his steps, he hoped they at least cleaned the table before they had to use it again.

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One more giveaway happened the day Sam could tell that Dean and Cas were holding hands at a diner. Not overtly, but under the table. They'd stopped to get a bite to eat after a case and talk about how things went, maybe discuss where to search for their next one. They'd been sitting at a small table, Dean and Sam across from each other and Castiel on an adjoining side.

Sam was mixing dressing into his salad, flipping everything around to make sure it was even when he happened to glance up and see Dean sitting motionlessly, burger forgotten entirely. He was staring blatantly at Cas, who was gazing out at the parking lot through the nearby window. Castiel was pensive, staring at nothing in particular that Sam could discern, and Dean had that soft look on his face with warm affection in his eyes. It was an expression Dean had very rarely ever used, and one that made Sam glad he had somebody to make him wear.

Even if they were being stubborn about letting him know things had changed.

Without noticing that he was being watched – really, Sam almost felt like he should be insulted at feeling so invisible– Dean apparently reached over to take Castiel's hand beneath the table, his arm inching closer and Cas's arm moving subtly.

Cas started to turn his head to look at Dean, and although Sam wasn't in his line of sight he still glanced down quickly, stabbing a cherry tomato.

Sam really wasn't sure why he didn't simply let them both know that he knew already. He told himself it was to let them be the ones to open up, but maybe (since it took Dean years to tell Sam that he was bi) it was curiosity to see how stubborn they could both actually be.

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Another time occurred two weeks later, when they separated to take care of a routine haunting and Dean and Castiel had gone off together to find the girl's grave to do a salt and burn. Sam felt like Dean expected him to protest more – oftentimes, distracting the ghost from its victim was the more difficult job, and two people together could do it easier – but he really didn't mind, as the ghost in this case was a twelve-year-old who probably wouldn't be much trouble.

At any rate, Sam was able to keep her away from her victim (the girl's mother, actually). Soon enough, the girl's fight was abruptly cut short as she went up in flames.

"Oh, my God," gasped the redheaded woman, totally devastated.

"I'm sorry," Sam said gently, putting a hand on her shoulder. "For… all of this." She nodded wordlessly, tears in her eyes. Sam set about fixing things in the living room, turning the reddish armchair back upright and picking up scattered books.

Dean and Castiel were probably going to be back any time. Sam helped the woman to the first-aid kit in the bathroom. He quickly and efficiently tied a sling for her broken arm, knowing it wasn't as good as what the hospital could do but enough for a stopgap. "Thank you," the older woman said, fiddling with the knot nervously when he finished.

"No problem." There was an awkward silence then.

Where the hell were Cas and Dean? Sam glanced toward the living room window and then grimaced, realizing he probably didn't want to ask that question. Hopefully, the grave had been farther out and it was taking longer for them to walk back or something.

The woman cleared her throat and took a seat on the armchair that Sam had righted. "So, do you guys… do this kind of thing often?" she asked uncertainly.

Sam sighed. "Unfortunately, yeah," he replied. "Never ends." He dug into his pocket to call Dean, but was interrupted by the sound of boots tromping up the porch steps. Finally.

When Dean and Cas walked in, both their hair was messed up just a little bit and they were walking with the awkward, self-conscious gait of sexually-frustrated teenagers trying to hide what they'd been doing.

Sam stifled a laugh. Did they actually think they were fooling him?

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Sam almost gave in and blurted out that he knew about things the night they were sitting around the table in the war room and watching a rerun of Breaking Bad on his computer. It was an episode he'd seen before, but Cas hadn't and (as far as he knew) Dean hadn't either.

Both of them seemed restless, making little motions as they watched the screen. Castiel folded his arms, then unfolded them, then rested his chin on loose fists, then folded his arms again. Dean cracked his knuckles one-by-one until Sam gave him his patented bitch face at the sound, and Dean stopped on the sixth knuckle.

Still, Dean was on edge, jiggling his right foot and resting his jaw in his hand. Finally, he shoved back from the table and stood up. "It's been a long day," he said, stretching his arms over his head. "I'm gonna… hit the sack here."

Inwardly, Sam rolled his eyes. He wondered how long it would take before Castiel would claim to be mysteriously exhausted as well. Ten-to-one Cas would exit stage right in ten minutes or less. Fifteen if he was really trying to play things cool.

His estimate ended up being too high by nearly triple the time; after a mere five minutes, Castiel yawned badly (faker) and blinked repeatedly. "I'm tired too," he announced, not quite meeting Sam's eyes. "I think I'll go to bed as well."

"All right," Sam said. "Good night, then. See you in the morning."

He watched Cas leave, thinking that his friend really needed to brush up on his acting skills if he was going to play a part in their FBI routine or people would figure him out pretty quickly.

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Everything came to a head three weeks after that, when Sam returned to the motel room with newspapers and coffee to see Castiel and Dean sitting pointedly apart from each other, Dean on one of the beds and Cas at the little table stiffly. An outsider would think that the two didn't get along at all with how much they were overcompensating.

If not for the fact that they both had wet hair from the shower and still-swollen lips.

This was crap. How dumb did they think he was?

Sam stopped next to the table and letting Cas grab the papers from between his arm and the coffees. Wordlessly he handed Dean and Cas's coffees to them, then got his own ready.

"Any other info on the case?" Dean asked, looking over Castiel's shoulder as he flipped through the first of the papers.

"Nope," Sam said, taking a drink of his coffee. "I did hear something in the elevator coming up, though."

Dean reached forward to point at a story in the paper, and his arm brushed against Cas's as he did so. They both jerked away from each other like they'd been electrocuted. "Sorry," Dean mumbled, faking discomfort.

Sam sighed. This needed to end now. "Fuck's sake, guys, stop with the act."

Cas did his little eye-squint expression. Dean stood and crossed his arms. "What the hell are you talking about, Sammy?"

"Stop." Sam was 100% done with their bullshit. "I know you're sleeping with each other. I've been trying to turn the other way, so you felt comfortable telling me in your own time, but I'm not an idiot here."

Dean shook his head. "No, we're—"

"We are," Cas cut him off. Dean reddened.

"Don't know why you couldn't just tell me," Sam said. "Not like you haven't been with guys before. You know I don't give a damn."

Dean's mouth opened and shut. "Cause… I don't know…" He leaned against the table with both hands, head down, before he looked back up. "Cause it's Cas. Thought you might think it was weird."

Sam shrugged. "Everyone, everyone, from angels to demons, has joked about you two. And, oh yeah, I have eyes. You guys have been together for years whether or not you admitted it."

Cas glanced over at Dean. "He has a point."

"I guess," Dean said. "Well, at least we don't have to play pretend anymore."

"I would hope not," Sam laughed. "Your acting was the worst I've seen in a long time. Cas, you especially."

"Had you fooled for a bit, though, huh?" Dean smirked.

Sam rolled his eyes. "Not even for one day."