"I think I'm insane," he says one day. He's never sure if he really meant it or not that first time he says it, really. It's comes more out of a sort of desire to cause something—panic or surprise or what-have-you—and it's only later that he realizes that somewhere underneath there the statement might be true.
"Well, I suppose everybody's a little bit insane," Remus says, the sentence seeming oddly wise. As a third year, he is still just young enough to be able to have that effect, before he is teenaged enough for it just to sound like him trying to seem deep and intelligent. "It's not just you."
The first thing about Remus that annoys Sirius (and the one that forever annoys him the most) is his ability to make everything seem reasonable and commonplace and ordinary, especially when Sirius wants something to seem special and exciting and different. "But this is different," Sirius insists, and then he stalks away, though he's not sure where to.
That's another statement he never really means at first; it's more just an immediate retort to use without thinking, and then he starts believing it because if everyone's a little bit insane, then it is not the same kind of insanity Sirius is sure he possesses. Remus is too sensible to be even the tiniest bit insane.
(Remus never mentions the conversation again, but Sirius knows he remembers. It's that look in his eyes as the war draws on and they're out of Hogwarts, years later.)
--
"I think I'm insane," he says. This time, it's to Bellatrix during some sort of family party at Grimmauld Place. It is the last time her duties (she calls them duties, but everybody knows what she means and she knows it too) don't interfere with her presence at these parties, and it is also the first and last time since Sirius was sorted into Gryffindor that she talks to him.
She smiles (her eyes crinkle, but the smile still looks sadistic and somewhat fake) and she says, "And what's wrong with that? You can accomplish so much more without the restraints of sanity." Then she walks away, off to make tittery small talk and keep up appearances, which is what the family parties are for.
If he was sane, he would have shivered. Instead, Sirius turns this remark over and over in his head, and eventually puts it away in the back of his mind, where it will not bother him.
(It's not a surprise—Sirius has always somewhat known his oldest cousin was insane; it's the look in her eyes. And it's that look in Bellatrix's eyes that causes Sirius to shudder, not what she says.)
--
"I think I'm insane," he says. He always feels insane after a battle, any sort of encounter with Death Eaters, but this time he states it. "I feel like there never was an edge to fall off of to begin with." Blood begins to run down his arm. Sirius ignores it. It's just red stuff.
Marlene McKinnon smiles up at him, and her teeth are too white and innocent compared to the blood on her face, and there's the look in her eyes. Usually Sirius thinks it's sexy but this time it's vaguely disconcerting, and he knows he should be feeling more than that. "What edge?" she says. (Sirius never figures out if she was joking or not, and the memory disturbs him for the rest of his life.)
She's killed three days later. It's the last time Sirius ever talks to her and the string something—he's not sure what it is—is hanging by is too thin for him to really register emotion or to care.
It's easy being insane. Sanity just hurts.
--
"You're—you're insane," Wormtail, the filthy traitor, gasps. The string's nearly invisible so Sirius isn't sure, but as the rat makes his escape, he laughs anyway, bitterly, and doesn't really care that he's getting dragged away.
--
"I'm insane," he says one day in a cracked voice, with no one to respond in the cell. He doesn't bother even laughing because it's old news now and there doesn't need to be an "I think I'm" anymore.
(When he's escaped, nobody seems to suspect anything but Moony. When you're insane, it's easy to be anything else.)
--
I don't like this much. I'm not sure, actually. I really hate the ending, though.