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The thing about a good prank, Leonard muses, is that you can really only pull it off once. Past that, the surprise is gone, and if you push your luck with the same stupid joke too many times you start to look like an asshole.
But three months into their friendship, when he realized that Jim was never going to stop slapping him on the back in greeting or encouragement every time they saw each other, he also realized that such physical, touchy-feely exuberance made Jim the perfect target. And since then he's been biding his time, waiting for the perfect moment and closely guarding certain knowledge of his past that might give up the joke before he can play it.
It's a little vindictive when it finally comes, he has to admit: things have been quiet for weeks, no signs of danger on what passes for the horizon out in space and minimal stress for everyone in the crew, Jim included. But it hasn't been so long since Jim last landed on his operating table, pale and bloody and barely breathing, for Leonard to have quite gotten on with the forgiving and forgetting. If he's going to keep waking up from nightmares of his best friend dead or dying, it feels like fair turnaround to give Jim a taste of the horrified concern that comes with seeing a loved one injured and suffering, for once.
(Only a little taste, mind. Leonard isn't so cruel as to forget the kinds of things Jim has already seen in his lifetime.)
There's no harm done, anyway. It goes like this, when it happens:
"Bones! Hey, Bones, wait up," Jim calls, and Leonard can hear him come jogging down the corridor to catch him up. He doesn't pause in his own stride because he does, in fact, have somewhere to be, but Jim—as usual—is undeterred. His hand lands heavily on Leonard's shoulder from behind, a solid clap followed by a tug meant to stop him in his tracks and turn him to face his friend.
Recognition of the opportunity, as perfect as it could ever be, flashes faster than conscious thought; Leonard reacts in the heartbeat it takes to spin around, popping his shoulder easily out of its socket. Jim must feel it move, because his face is already twisting up in confusion—rapidly shading toward alarm—as Leonard reaches up to clasp at the arm now hanging limply by his side.
"Jesus fuck," he gasps. He reels backward, and Jim lifts his hand away like he's been burned. Leonard hisses through his teeth, clutching the limp arm tighter against his body; Jim takes a step closer, his hands hovering now with the hopeful need to touch, soothe, help in some way.
"Oh my god, Bones, are you—shit, I'm sorry, I didn't mean to—"
"Don't," Leonard warns as Jim's hands descend. He bites his lip hard, wincing, and really plays it up with a pained groan. He's never been much of an actor, really, but it's enough to convince Jim, who backs off immediately.
"No, right, of course. What—is it broken?"
"Dislocated," Leonard says, which eases some of the stricken look from Jim's face. He seems to flounder for another moment, his eyes still bright and wide with shock, before the captain's persona slides into place.
"Okay. Okay, sickbay. I'll call and tell M'Benga we're coming."
"Ah, don't bother." Leonard gives his arm a quick shove, and the joint pops back into place. He flexes his hand, grinning, and jabs affectionately at the ticklish spot on Jim's side. "I'm just messing with you, kid, I'm fine. Should have seen your face, though."
Jim blinks, opens his mouth, blinks again. When no words seem forthcoming Leonard nudges him with an elbow then slings his arm around Jim's shoulders, still grinning.
"I really got ya, huh? Can't believe you thought it was broken. I promise you you're not that strong," he chuckles. "Now, Spock, maybe..."
Jim lets out a reluctant laugh at that and softens against Leonard's side. "You scared the shit outta me, Bones. How'd you do that?"
"Grew up around horses," Leonard says. "Got thrown a few times—it happens—and injured my shoulder once. I've been able to pop it in and out of the socket ever since."
He doesn't need to look to know that Jim is smiling at that, probably still half-reluctant but fascinated. That assumption is borne out a moment later when Jim huffs quietly.
"Cool," he says. "Gross, but cool. Any other neat tricks I should know about?"
"Nope," Leonard says innocently. He catches a sidelong glance from Jim and laughs. "Really, Jim, it's just the one. I promise you, your body is more fucked up than mine."
"Thanks, I try," Jim says wryly. "You've been able to do that since you were a kid, huh?"
"Yup."
"I've never seen you do it before."
"Nope."
"You've been planning this for a long time."
"Oh, yeah. Years. How long have we known each other?"
"Ten years next month," Jim answers without hesitation, and then scowls. "Ten years? Bones."
"Yeah, Jim?"
"You're an asshole."
"Now that ain't news," Leonard says, smirking. "Figure you've known that about me since the day we met."
"Yeah," Jim agrees, but there's no heat in it. It's warm and fond, a honey-soft sigh. He elbows Leonard in the ribs as soon as he's done being sappy, though. "Scare me like that again and I'll dock your pay."
"Yeah, yeah," Leonard dismisses. "We good?"
He doesn't really need to ask, but Jim humours him with a reply anyway: "Always."