Finishing off most of his chocolate, Sirius adopted another thoughtful express as he gestured with the remainder of the sweet. "Well... I would tell you, but..."
But that wouldn't be as much fun as publicly embarrassing Remus. Not much, mind, but a little -- or at the very least making the other bloke think he'd done something barmy in front of the entire commons.
"But I have to get you drunk before I can tell you," Sirius finished, feigning complete seriousness. "You'll never agree sober. And it's something you need to know if you want even a ghost of a chance to stop me, so... you've got no choice but to get drunk, mate."
That concocted, Sirius leaned back slightly on the bench again with a satisfied sort of air and waited for Remus to decide how to respond.
Whatever he came up with, Sirius was set on this. It was going to be his entertainment for the day.