The original Broadway production of "40 Carats", adapted for the English language in the USA by Jay Presson Allen from the French play "Quarante Carats" (1967) written by Pierre Barillet and Jean-Pierre Grédy, after two previews, opened at the Morosco Theater in New York on 26th December 1968, and ran for 780 performances, before it closed on 7th November 1970.
Gene Kelly said of working on this film: "I couldn't see myself declining the opportunity to work with Liv Ullmann, an enchanting actress. I wanted to work in a film with that wonderful actress and that was enough". About playing a smaller supporting role rather than a leading part, Kelly said: "That's not the point. It's good material and it's time we in Hollywood got away from this pretentious business of labeling the appearance of a star in a small part as a cameo, as if excusing it".
Julie Harris won the Tony Award for Best Actress in 1969 for her role as Ann Stanley in the Broadway play, played here by Liv Ullmann. Zsa Zsa Gabor succeeded Harris on Broadway season on 6th July 1970. Harris had played the role there since late December 1968.
The character of Ann Stanley was supposed to be 40 years old. Liv Ullmann, who played her, was 37. The Peter Latham character was supposed to be 22. Edward Albert, who played him, was 22. The age difference between them was 18 years. Trisha Stanley, Ann's daughter (Deborah Raffin) was 17.