The Experiment (2001)

Brilliant German psychological drama about a group of 20 men who volunteer to take part in a 14-day prison experiment. 10 men play "prison guards" and the other 10 play "prisoners". They are watched via cameras by a group of doctors at all times.

Only things begin to go wrong after 36 hours and the "prison guards" (particularly one) begin to take the "game" a little too seriously...

Won't say anymore. Great stuff.