Beast Must Die - Amicus Classics 
A rich sportsman invites eight guests to his house with the knowledge that one of them is a werewolf. He has devised a game of suspense wherein the guests must guess whom is the wolf. At the full moon, the game will end as the werewolf unwittingly reveals himself, and the others hunt him down to add his pelt to the sportsman's trophies. The film features a special "guess-who" sequence at the end, in which the viewer is given thirty seconds to decide "who done it."