A/N: Sorry these have all been narrative so far; as I get more familiar with this fandom and pin down the characterizations I will try for a more active voice. :)
C: Curiosity/Capsize
Curiosity is not one of John's more prominent traits. He's learned to roll with the punches, and in the military he learned to shut up and follow orders. Both those skills are vital for getting along with Sherlock.
But when his daily life starts to involve casual government abductions, not to mention body parts turning up in the vegetable drawer, John's sense of curiosity is piqued. And when he realizes that Sherlock is so accustomed to his presence that John can get away with rocking the boat a little, John starts getting a little impertinent with his questions.
Why have you let Mrs. Hudson in, when you push most people away? (The answer turns out to have very little to do with Mrs. Hudson and a great deal to do with Sherlock, although her meringue cookies make it in there somewhere.)
What'd that bloke Sebastian mean about everyone hating you at uni? (Turns out, that. Precisely and exactly that. Sherlock says he wasn't as careful about other people's feelings back then as he is now. John can barely imagine.)
Be straight with me about the drugs, just once, give me an honest answer. (A failed experiment in the university years; failed not because he hadn't learned from it, but because of the unforeseen result of a crippling addiction. Sherlock won't acknowledge the correlation between the timing of the experiment and the crippling boredom and isolation he felt at uni, but he doesn't have to. John sees it anyway.)
What's the deal with you and Mycroft anyway? Tell me it's not just petty sibling stuff. (This answer takes some work to get out of him; Sherlock is slow to open up about Mycroft, but one late night he does and it's John who backs out of the conversation two hours later. John says forget it, he's changed his mind, he no longer wants to know the things Mycroft did as a teenager because he still has to look the man in the eye occasionally.)
And why, exactly, do you need me around on cases? (To toss him pens, call him brilliant, and insulate him from the Andersons and the Donovans of the world, in that order. Apparently Sherlock considers pens a high priority.)
Why does everyone who knows you think we're a couple?
C'mon, you're not going to answer that one? Why do people think we're a couple?
(Sherlock walks up to him, right up to him, past punching distance into kissing distance but does neither of those things. Just stares at him for a moment, that really high-intensity focused look he gets sometimes, and says Because. He turns away and walks out of the room, down the stairs, out the door. John has to reorder his mental map of their relationship. He's been indulging his curiosity freely for awhile now, but he knows now that some things are still off limits. John knows he's found the one thing that can capsize them.)