Murder Mansion

Six people receive an invitation for a party or a celebration at the host's mansion. As they arrive at their destinations, the host locks the mansion down and says that unless and until they watch a movie and figure out who the murderer is, they will not be allowed to leave the mansion.
They watch movie. Movie stops halfway. When they come out of the room, they find the host dead, and the rest of the movie reel burning to ashes. No one except the host knew how to exit the mansion. Now the host was dead. They must figure out who killed the host and solve the murder in the movie to escape the mansion. Till then, they have the killer on the loose....