It was agreed Cary Grant would keep all of his clothes on when he took a shower, as he was nearly sixty and slightly overweight. However, they then decided the scene was funnier that way.
After finishing this film, Cary Grant was quoted as saying, "All I want for Christmas is to make another movie with Audrey Hepburn." Although it never happened, Grant was offered the role of Higgins in My Fair Lady (1964), and requested Hepburn as his co-star in Grand méchant loup appelle (1964).
According to Audrey Hepburn, the scene where Regina spilled ice cream on Alex's suit was based on a real-life accident where Hepburn spilled red wine on Cary Grant's suit at a dinner party.
Due to the suspense, the presence of Cary Grant, the structure of the screenplay, and the frequent plot twists, many people believe this was a Sir Alfred Hitchcock film. Hitchcock was not involved in the making of the film at all. This confusion has prompted fans of the film to call it "the best Hitchcock film Hitchcock never made."
During the dance game in the nightclub, Audrey Hepburn's husband, Mel Ferrer, can be seen briefly in the background, smoking a cigarette.