Sirius is more than happy to troll Remus. It sounds like an awesome plan. That is, until he opens his mouth and hears his own voice coming out of this body. "Remus!" Damn it, that will make it harder to fool Remus.

He deepens his voice and puts on a stern expression. He noticed he was suddenly wearing Hogwarts Professor robes. He can so prank Remus with this. His voice sounds hoarser, more gravely when he talks again but if Remus is listening closely, he might realize it's Sirius. "Master Lupin! There you are! Playing hooky from school to go party, how very brave of you!"