Sirius' laugh was very humorless. "That right, Severus?" he said quietly. "Well, let me remind you it was only a few years ago that you tried to give Remus and I up to the dementors even though you knew we were innocent." His voice went low and harsh as he spoke the last few words, and something strange glinted in his eyes.

He leaned forward, grasped Snape's collar, and said "Have it your way, then. But if anything happens to Remus-- if anything goes wrong with the potion or he doesn't get it in time or he gets sick from working too much-- then I will fucking kill you. Understand?"