Joan Fontaine got the lead role of Tessa by a lucky chance. One day, she was having lunch at Romanoff's in Hollywood, with her husband, actor Brian Aherne. The two had just flown in by airplane from their grape ranch in Indio, California, and Fontaine was in a leather flight suit with her hair done in pigtails. Director Edmund Goulding walked into the restaurant, and stopped by their table to say hello to his good friend Aherne. Goulding complained that he was having trouble casting a lead actress for his next movie, "The Constant Nymph." Although he had considered Joan Leslie, she was wrong for the part. And, Goulding explained, "Jack Warner wants a star in the lead, but she has to be consumptive, flat-chested, anemic, and fourteen!" "How about me?" said Fontaine. "Who are you?" asked Goulding, not recognizing the freckled girl in pigtails sitting next to him. "Joan Fontaine," said the actress. Goulding looked startled. "You're perfect!" Fontaine was signed for the part the next day, and later called it "the happiest motion-picture assignment of my career."
Joan Fontaine often said that Charles Boyer (her costar in this film) is her favorite leading man out of all leading men she worked with. Joan Fontaine said that working with Charles Boyer was pure joy.
For a long time, this film wasn't available because the rights to Margaret Kennedy and Basil Dean's play, Margaret Kennedy's novel and the original film rights owned by Gaumont British all became separated at different times in the late 1950s and eventually expired. The script and screen version used in 1943 was a combination of novel and play. Separate contracts were negotiated at that time with both authors. The Kennedy Estate had no objection to the film being shown, but it would have required expensive legal intervention to resolve the contractual situation. Somehow this must have been straightened out as the film is now available. The Library of Congress Film Archive restored it and it premiered at the Turner Classic Movie Festival in spring 2011 and aired later on the TCM cable movie channel.
The "inside" of the Sanger's house (Tessa's house) was previously used for the "inside" of the house of Brewster Sisters in Frank Capra's Arsenic and Old Lace (shot in 1941, released in 1944). The Constant Nymph was made in 1942 after the shooting of Arsenic and Old Lace was finished.