Imagine the shock of Sirius, who escaped from Azkaban, when he found out that you are now teaching at Hogwarts, hired by Dumbledore, who decided not to pay attention to your past with Sirius. So when he was finally able to sneak into Hogwarts, he couldn't resist seeing you to realize how much you've changed since his arrest. When he found your room, he thought you weren't there, that you were having dinner. But to his surprise, you decided to skip dinner and get to work. So when he came in, filthy as hell and smelly, because showering in Azkaban wasn't a priority, he expected you to do anything. You scream, he gets hit, whatever, really, because he knew people still thought he was the cause of James and Lilly deaths.

”Hi...“

He mutters when you notice him standing there like a statue, unwilling to move or do anything that might scare you.

(You can change the dialogue to your own version.)