Murder by Natural Causes (1979)

The film opens as successful mentalist Arthur Sinclair (Hal Holbrook) is talking on the phone with his young wife Allison (Katharine Ross). She wishes him luck as he departs to give an interview. The call ends--and she's in bed with another man: Gil (Barry Bostwick). Gil is a trying actor in love with Allison, who is planning to have Gil kill Arthur by scaring him to death, as he has a serious heart problem. Their plan is to have Gil storm the house pretending to be a thief and point a gun at Arthur, who would die from the shock. The fateful night comes and Gil storms into the house, but Arthur sees him before he can reach the gun. Gil presents himself as a journalist there to interview Arthur. He and Gil have a hearty conversation and it turns out that both men have something on each other. Gil knows that Arthur is a fake and that he somehow knew the information about a killer in a different town and Arthur knows that Gil is not a journalist. Gil is not panicked by that and he uses Arthur's competitive behavior to make him do 50 push-ups which tires Arthur's heart. Soon Gil reaches the gun and points it at Arthur, who has a heart attack and dies. When Allison comes home she and Gil kiss but are scared by Arthur, who isn't actually dead; he was only pretending. He knew about Gil and Allison for a long time and knows that he is out to get them, but suddenly Arthur's lawyer and best friend George (Richard Anderson) comes into the house and it turns out that he is Allison's real lover and Gil was just there to do the dirty work. George shoots Arthur and he and Allison depart. Some time later, Allison returns to the house to find Arthur's body gone. It turns out that Arthur knew about George too. Arthur points the gun at Allison, she asks him what he will do, and he replies: "Why don't you read my mind?"