From script to film:
* Seven studios rejected the original screenplay. Peter Stone turned it into a novel which was serialized in Redbook, and it was then turned back into a screenplay -- which had interest from all 7 studios.
* Grant was uncomfortable playing a romantic lead with an actress young enough to be his daughter. He was appeased when jokes about the age difference were added to the script and it was made clear that it was Regina who was pursuing him, not vice versa.
True to life:
* According to Audrey Hepburn, the scene where Regina spills ice cream on Adam's suit is based on a real-life accident where Hepburn spilled red wine over Cary Grant's suit at a dinner party.
* The character of Peter Joshua was named after Stanley Donen's two sons: Peter and Joshua.
* The young man in the Embassy elevator telling the story about the poker game is screenwriter Peter Stone - with the dubbed voice of director Stanley Donen.
* The voice of the Marine in front of the Embassy is dubbed by screenwriter Peter Stone.
* When first released, Hepburn's line "at any moment we could be assassinated" was dubbed over to become "at any moment we could be eliminated" due to the Kennedy assassination. Subsequent releases have "assassinated" restored to its place in the film.
* In the scene where Audrey Hepburn is smoking a cigarette alone in her empty apartment and Cary Grant enters, the backs of his ears had to be covered with masking tape since the backlighting made them appear red.
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