Guy Haines
Guy is the protagonist of the movie. He is a tennis star on the amateur circuit, and is married to a woman he can no longer stand to be around because she is vulgar and promiscuous. He has fallen in love with the elegant and charming daughter of a Senator, and wants to divorce his wife so that he can marry her instead. Guy does not have much backbone when it comes to standing up for himself or for what is right, and this is what gets him into the murderous agreement with Bruno Anthony. Rather than telling Anthony that he wants nothing to do with the plan, he nods and smiles his way through their conversation until he is able to disembark the train.
Guy realizes that he has dug himself a deep hole in becoming acquainted with Anthony and seems to live a charmed existence in that he essentially gets what he wants - his wife out of the way - without ever getting blamed for her murder. Although he is technically complicit in the plan, Anthony would probably have killed her anyway because of his psychopathy. Guy ends the movie in a strong position, with Anthony identified as the killer, and Guy free to move on with his life and the woman he loves.
Miriam Haines
Vulgar and known to sleep around, Miriam is far from the loyal wife, and although not a main character in the movie in terms of the audience getting to know her as it progresses, she is pivotal in that she is the victim of the murder. She is a difficult woman and has resorted to blackmailing her husband to get her own way, and whose personality seems to bring out the worst in her husband; however, this is no reason for her to be the victim of a murder and she has the misfortune of encountering a man who is a psychopath looking for a way in which he can murder his father and get away with it, and who sees her as a sort of quid pro quo that will enable him to do that.
Bruno Anthony
Anthony is a psychopath. He does not mind killing; in fact he seems to rather jump at the chance. Rather than killing his father, who is the real target, he decides that he should kill Miriam as a favor to her disgruntled husband because he will then be owed a reciprocal favor, and will therefore be able to engineer his father's murder without getting the blame for it. He is both deceptive and dangerous. He is generally one step ahead of Guy. Had the carousel remained in normal operation he would have likely overpowered Guy in a fist fight and killed him, or at the very least, been able to convince the police that he was trying to prevent him from fleeing.
Anne Morton
Anne is one of two Morton daughters; erudite, glamorous and elegant, she is everything that Miriam is not, and everything that Guy is looking for in love and in life. She is supportive and loyal, believing him immediately when he tells her that he was not the instigator of the murder plot, and that he did not murder his wife. She is also a pro-active woman, deciding to visit Anthony's mother and tell her that her son is a murderer, believing that appealing to her woman to woman will be more effective than going along with the plan. She is also very intelligent and realizes immediately the connection between Anthony and Miriam after seeing him try to strangle a woman whilst staring at her sister at a party.
Barbara Morton
A peripheral character, Barbara is also rather pivotal in that it is her similarity in appearance to Miriam that gives her sister something of an epiphany; Barbara is understandably spooked by the fact that Anthony was looking at her whilst he was trying to strangle a woman at the Morton's party; this would be disquieting to anyone, but it is her resemblance to Miriam and the jarring effect it seems to have had on Anthony that gives Anne her "aha!" moment and allows her to connect the dots between their murderous party guest and Guy.
Senator Morton
The Senator is the character who tells Guy that Miriam is dead, although Guy already knows it. He is vaguely suspicious that his potential son in law has had something to do with the murder but not sufficiently convinced of it to take any action. He is not so much a character in the novel as a backdrop to Anne, explaining her upbringing and the reason for Guy's attraction to her.
Park Employee
The park employee is un-named but is nonetheless important because he is the catalyst for the action at the end of the movie and the loose ends of the plot being neatly tied. He calls the police to the park with the intention of reporting Anthony. Their assumption that he was identifying Guy led to their shooting of the carousel operator, which indirectly leads to the accidental death of Anthony and Guy's freedom.
