This is kind of a re-vamping/re-imagining of an old headcanon *Warning for sexy-ish stuff and teacher-student fantasy (eventual role play)

He realizes Remus is the same old Moony who singed off his eyebrows (twice!) in fourth-year Potions and sticks his icy toes under Sirius’ thighs on Winter nights. Yes. Same old Moony. But when he learns of Remus’ appointment as Hogwarts professor, Sirius’ interest is... piqued, you could say.   When he sits tucked away in the far corner of Remus’ classroom, Invisibility charm firm around him, and actually watches a lesson? He finds himself entranced by the man’s voice, calm, deep, authoritative, and slightly hoarse from having taught the lesson three times already that day. Entranced by his broad white smile, the crinkle of his crows feet, the surety of his movements, the strong hands motioning this and flicking spells easily into that, the figure he cuts in the new fitted cloak and trousers he’d splurged on for the job. It was almost obscene how gorgeous Moony looked up there, and Sirius wondered if the students knew.

When Remus stepped tiredly through the door that evening, returning to the off-grounds flat he shared with Sirius, Sirius was upon him almost instantly. His stooped shoulders straightened, his eyebrow quirked, his lip smirked. He beheld the dark, heated wonder in Sirius’ eyes, the pink on his cheek. Before he could press out a husky “Sirius, what-” Sirius had already draped himself over the other man and pulled him close to kiss at his jaw and bite at his ear and whisper a “Don’t you know, Professor Lupin?” And the way Sirius bit his lip and the way his heart beat fast against Remus’ when their chests pressed together meant that Remus did know.   Sirius had shown him, with the wideness of his pupils and a look that said What are you going to do about it? and Please, Professor, I need you badly, badly all at once.