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.
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.
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.
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."
The United States Production Code, which was only very slowly being eroded during the period when this movie was being made, stipulated that "the subject of abortion shall be discouraged, shall never be more than suggested, and when referred to, shall be condemned. It must never be treated lightly, or made the subject of comedy. Abortion shall never be shown explicitly or by inference, and a story must not indicate that an abortion has been performed, the word 'abortion' shall not be used." Ann Lake speaks quite forthrightly and neutrally with Superintendent Newhouse about the fact that she had considered having an abortion, though she did ultimately choose to give birth to her daughter, Bunny. Contrary to the restrictions of the production code, Ann even uses the word "abortion" during that conversation. A 2015 New York Times article about this movie by critic J. Hoberman said that this movie was likely "the first studio release to employ the forbidden word 'abortion'." (The movie Peyton Place (1957) used the abortion word when Dr. Swain admitted performing one on a victim of incest, and then showed his shameful regret after he said it.