... gives Willoughby that same stare that Snape gave Potter so often, and Willoughby reacts. The question is what would that reaction most likely be?

I think he would grasp the doorpost with one hand, staggering backward as he raises the other hand weakly in defense from the stare. He stammers incoherently, turns and lunges for his carriage, falling against its side. His footman has to assist him - lift bodily more like - into the coach. Cowering in a corner as the coachman cracks the whip to start the horses, the last anyone sees of Willoughby is one horrified eye peaking over the window frame. Brandon laughs Snapily. Fade to black.