Sleuth (1972)

Andrew Wyke is a famous and successful author of detective novels. Milo Tindle comes to him with a strange request, that Mr Wyke divorce his wife so that Tindle can marry her. Mr Wyke is not particularly perturbed by this, he and his wife have drifted apart, and he is having an affair with another woman anyway, but uses the meeting and Mr Tindle's request as a chance to play a game, a game with potentially deadly consequences.