Severus almost flinches at that. It's been years since he's heard any of this - students surely have a vast array of giggling and spiteful commentary about him but no one dares breathe a word of it where he can hear, and he doesn't miss it. He moves to shove him off but even though Severus has a few years on him, Sirius is still stronger-- his right hand wedged against the Gryffindor's chest is perfectly poised to coax a spell out, something debilitating, but he reels the impulse in.

Barely.

(He doesn't know why.)

"So sorry you're lonely, Black," he hisses, low, "But this is--"

Presumably Sirius isn't going to actually let him get a lecture going, but he can always go on.