Snapedom

I'm sure Severus' insistence that Remus drink the potion immediately is mostly about Severus not trusting Remus to drink it at all and thus becoming a risk to the school populace in general and Harry in particular, as eventually indeed happened on the night after Buckbeak's intended execution. But the impression of fear in that scene is from the way Severus backs out of the room. Severus should have realized Remus wasn't going to take the potion with him there so a more rational approach would be to just leave and come back later, after Harry was gone, to check whether Remus took the potion (and force it down his throat if he hadn't yet, or get Dumbledore to force the issue if needed). So either Severus fears to turn his back to Remus or he wants Harry to know Remus is untrustworthy.