Brandon Shaw
Brandon is the narcissistic co-host of the party supposedly in honor of his former classmate. He is the mastermind behind the murder, driven to commit the act of violence with an artistic and passionate fervor. Brandon truly believes in his own superiority, and seeks to play God in relation to the other people in his life, orchestrating social situations, matchmaking, and causing psychological mischief wherever he can. He gets away with his more unsavory characteristics because of his immense charm. People scold and disapprove of him, but he still manages to maintain their trust, even though he is not worthy of it. Brandon is also defined by his desire to put one over on people with a pronounced flourish, a flourish which ultimately leads to his own downfall. Rather than hide the body, for instance, Brandon decides to host the dinner party around the box that contains the body. Additionally, he keeps the murder weapon around, and even ties up Mr. Kentley's books with it at the end of the night. He is controlling with the more sensitive Phillip, and does not listen to Phillip's anxieties.
Phillip Morgan
Phillip is Brandon's accomplice, who goes along with the plot, but exhibits a number of reservations. While he is also seduced by the darker allure of Nietzschean ideals, he is less intent on following through with the actual murder than is Brandon. Rather, he submits to Brandon's will, and struggles to keep it together in the wake of their evil deed. Highly strung, with an intense feeling of guilt rising inexorably to the surface throughout the night, Phillip spends the night drinking and compulsively playing the same song on the piano to calm his nerves.
Rupert Cadell
Cadell is the former prep school housemaster of Brandon, Phillip, Kenneth, and David Kentley. Through Rupert's influence, Brandon and Phillip became exposed to the ideas of Nietzsche and his philosophical teachings on intellectual superiority, and the absolute rights of those superior few to do anything, including murder others. He now works in publishing and is characterized as a curmudgeonly iconoclast and "radical," who does not suffer fools. While Rupert admires Nietzsche intellectually, when he realizes that his philosophies have inspired an actual murder perpetrated by his former mentees, Cadell is absolutely horrified and scolds his former students for their cold-blooded cruelty.
David Kentley
A kind-hearted schoolmate of Brandon, Phillip, and Kenneth. He is set to marry Janet, but Phillip and Brandon kill him at the start of the film. A tennis player, he is not very intellectual.
Janet Walker
Janet is one of the guests invited to the party in David's honor, a writer for a beauty magazine, and David’s fiancée. Their relationship becomes a source of tension and ultimately suspicion as Brandon seems unusually intent on trying play matchmaker between Janet and Kenneth in David's absence. Janet is witty and charming, but also chatty and insecure, often making jokes. Her relationship with David, she tells Kenneth, allows her to be herself in a way that she hadn't been able to before. She is depicted as fickle and perhaps a little weak-hearted, unable to choose between Kenneth and David.
Kenneth Lawrence
Kenneth is Janet's former boyfriend and former close friend of David. He is affable and kind-hearted, clearly still smitten with Janet.
Mrs. Wilson
Mrs. Wilson is Brandon's talkative and pro-active housekeeper. She is exasperated by all the peculiar demands on her caused by Brandon and Phillip's party, and expresses this frustration to Rupert Cadell, on whom she apparently has a crush. She plays a pivotal role in revealing the crime when she mistakenly hands Rupert the wrong hat.
Mr. Kentley
David's serious and elegant father, an apparently wealthy man. He is good-natured and interested in a number of books that Brandon has invited him over to look at. Compared with Brandon, Phillip, and Mr. Cadell, he is earnest and idealistic. While they all believe in the philosophy of Nietzsche, Mr. Kentley insists that Nietzsche is inhumane and presumptuous, and to believe him is to believe in one's own god-like status.
Mrs. Atwater
Mrs. Atwater is David Kentley's aunt. She is portrayed as pleasant but somewhat empty-headed. She has an investment in astrology, which she discusses with enthusiasm. She also reads Phillip's palms, telling him (to his horror) that his hands will become famous.
