Well, here's my first try. Hopefully something a little more independent of a specific episode will arise sometime soon...If this attempt is at all successful.
Criticism is just as welcome as praise- (here's my shameless plug:) Please review.
It was as if all Sam's fears had come true in less than 72 hours.
Not only did a shape-shifter decide to steal his brother's face, but it was taking his personality. It had been hard enough to watch 'Dean' walk around with his brother's memories, but his fears... Sam had always known Dean must have doubts, but he kept them so deeply buried that Sam never really gave them consideration. He knew it wasn't his brother emotionlessly rattling off Dean's hidden fears like a shopping list, but some part of him cringed to know that the thoughts existed somewhere in Dean's mind.
It hit him like a brick wall. Yeah, he'd left, but-- It didn't hurt Dean. Or, it shouldn't have- not logically. Dean was the rock, the impassively cool 'I accept my fate' guy whose feelings never got hurt. Even as the shape shifter in front of him leaned over to talk about their Dad, Sam could only picture Dean, his Dean, tearfully confessing how abandoned he felt, how scared he was of being left completely alone. That it would probably never happen made it that much scarier, and that much more painful for Sam to fathom. It took every ounce of his psyche to keep asking where Dean was, and to keep denying to himself that this wasn't him.
But what if his Dean was dead? What if this thing had killed him, and this would be the last Dean in any form he ever saw? Sam forcibly shook the thought- he would know if Dean was dead, and the shape shifter would have no scape goat if he was. Without concluding what happend to Dean, it had said something about Becky, then tossed a heavy sheet over him.
Sam had had a brief, renewing time with the real Dean, which was enough to remind him of the difference between the two. He glanced over at his brother as they jogged towards Becky's, and found it suddenly hard to imagine him being afraid of anything. Which is probably exactly why he acts so tough, a reluctant voice reminded him.
It got harder when he had to hit him. They always sparred as kids, but the initial crunching contact was still jarring and frighteningly real. Like a heartbeat, Sam had to remind himself regularly it wasn't his brother, and forcibly so at the sound of its voice. If the shape shifter kept his mouth shut, it was much easier to pummel him.
When 'Dean's' hands wrapped around his throat, Sam couldn't help but feel he had slipped into some horrific childhood nightmare. A dry sense of humor told him it was a relief from the nightmares about Jessica, as he quietly began to resign to the seeping blackness. The nightmarish scene progressed in sudden flashes, beginning with gunshots, and ending with blank, open eyes that stared back at him. Dean's eyes. A million half-realized fears and broken moments of worrying about his brother came crashing into one definite, concrete reality. Sam broke. Momentarily, everything came utterly undone around him as he stared at the one thing in the world he had never thought he could loose. He felt hands on his shoulders, but tried to ignore them. He couldn't come back- Dean was staring at him, with every last drop of fire and life sucked clear out of his eyes, and Sam could not even consider trying to move forward from the captivating horror of it. This was Dean, who had been unfalteringly there since he was born- who had softened the harsh reality of what was lurking under his bed just enough for an eight-year old to deal with, who had ruffled his hair, poked him and called him 'Sammy' just to get a rise out of him, but at the same time threatened anyone in school who looked at him the wrong way. All that being gone was staring him straight in the face, and everything was plummeting inescapably downwards. His brother was gone. Dean wasn't coming back. Dean, the same face who now moved in beside the passive eyes, to reclaim his necklace and snatch Sam back with a glance, was--- not dead. Very not dead, but very in shock. The sheer, sudden rejoice in Sam's eyes showed just how close he was to bowling his brother over with the largest hug he could muster, but Sam restrained. He recognized Becky's arms around him, and shook his reverie to recall the shape shifter- and the flash of anger, and the sudden awe at how calm his brother looked.
Dean, in fact, continued to look calm, even as Sam glanced over at him behind the wheel of the car. Boggled, Sam couldn't help but ask him about it.
"How are you doing that?" Dean cast him a nonplussed eyebrow.
"That. Being so calm."
"Well," Dean explained, as overly-slow as if he were explaining something overtly obvious, "I call it 'breathing'. It gets the oxygen to your brain, tends to keep you alive and kicking-"
"You know what I mean." Sam grumbled, annoyed that his brother was able to be sarcastic when he was still shaking imperceptibly. Dean chuckled.
"I guess I just try not to think about it," he answered more seriously.
"Not think about it? You just shot yourself."
"That's what I try not to think about." Sam shook his head, gazing back out the passenger side window in the usual silence that followed the brief wind-down pep talks after a fight. After a few minutes, Dean chuckled again, taking advantage of the limited seat space to ruffle his brother's hair and revel in his aggravation.
"You're such a dick." Sam growled, and Dean beamed proudly.
"That's what I'm here for."
Damn right, Sam thought vehemently, with a small sense of self contentment. And you're not allowed to leave.