Green Room (2015) written and directed by Jeremy Saulnier
A down-on-their-luck punk rock band gets possibly the worst gig ever--they play an afternoon set at a white supremacist bar. They don't know what their getting into, of course. The guy who arranges the gig just tells them not to talk politics. The band is more interested in drinking and goofing around, which they do until one of them witnesses a killing. They wind up locked in the green room of the bar and have to try to negotiate their way out. Things do not go well. The bar is owned by Darcy (Patrick Stewart) who is the sort of cool, collected, and ruthless man no one should try to outsmart.
The movie is very tense. The initial ten or fifteen minutes seems like a road movie for the band but the road ends pretty quickly at the bar. By that point they are established enough as characters that viewers care for their plight. The band struggles to figure out the situation and find some way out, either by talking or sneaking away. The situation doesn't go well for anyone. Enough little twists pop up to keep things engaging and more believable and more horrifying. The violence is very intense when it happens and is very gory.
I wouldn't call this a fun movie but it is engaging and a great example of a "siege thriller." With the graphic injuries, it's not for the feint of heart, either.