Double Indemnity (1944)

This is a film on the American Film Institute's list of the best 100 American films ever made.  It is particularly interesting because there is not a really likable character in the movie, with the possible exception of the Edward G. Robinson character, who is a bit of an unpleasant bulldog of an insurance salesman, but he has a nose for the truth and he manages to ferret out the heart of what happened.
Walter Neff is a door-to-door insurance salesman ably played by Fred McMurray who gets hoodwinked into an affair and a murder by Barbara Stanwick in order to collect on an insurance policy and divest herself of an unwanted husband.  Neff is easily convinced to participate and he quickly comes up with a plan--so he is not so innocent as he might appear--but as the story unfolds it becomes quite clear that the charming Ms. Stanwick is out for herself and she is not going to bring poor Walter along.  The movie has a great cast, a wonderful script and is the result of stamina on the part of the director, Billy Wilder, because it took him eight years to get the movie approved by the professional standards board that ruled Hollywood movies from the mid-1930's going forward.  Well worth watching, especially for film buffs.