Remus is chastising him, but it doesn't quite have the expected effect, considering the smarmy little smile on Remus' face that Sirius can clearly see reaching his eyes.

"You've caught me," he says, with mocking apology, shaking his head and attempting to look as chaste as he can manage, which is, of course, not very much at all. "Right then, little Moonlet, go on and finish that up then, you're nearly there," he says, grinning again and straightening up again.