It was Keir Dullea's performance in this movie that led to Stanley Kubrick choosing him to play his most famous role of Dave Bowman in 2001: A Space Odyssey (1968). He didn't even have to audition; Kubrick simply asked him if he wanted the role, and he said yes.
Keir Dullea and Carol Lynley discovered that they shared a strong Irish heritage, but, more important, that they were distantly related and still had relatives in Waterville, County Cork.
Columbia Pictures wanted producer and director Otto Preminger to cast Stella Stevens as Ann Lake, who was eager to play the role, but Preminger insisted upon using Carol Lynley. Stevens and Lynley would go on to co-star with each other in The Poseidon Adventure (1972).
At one point, Otto Preminger considered making the police superintendent a younger man and had Robert Shaw in mind for the role. Laurence Olivier, who played the part, disliked Preminger intensely and sometimes said that his wife, Joan Plowright, kept him in order by threatening to invite Preminger to stay with them as a house guest.
Noël Coward didn't think much of Keir Dullea as an actor. Reportedly he walked up to him on the set one day and whispered in his ear, "Keir Dullea, gone tomorrow."