Cary Grant had announced his retirement from acting in February 1953, stating that, since the rise of Method actors like Marlon Brando, most people were no longer interested in seeing him. He was also angry at the way Sir Charles Chaplin had been treated by the HUAC. He was lured out of his retirement to make this movie, and thereafter continued acting for a further eleven years.
In the scene on the float platform, Danielle makes a point of how much younger she is than Frances. In fact, Brigitte Auber was 4½ years older than Grace Kelly. Danielle is referred to as being a "teenager," but Auber was actually 30 when the film was made.
John Robie mentions that, as a youth, he was in a trapeze group that traveled around Europe. In reality, Cary Grant was in an acrobatic troupe that toured around Europe--and eventually brought him to America--when he was young.
Director Sir Alfred Hitchcock asked if chase scenes could be filmed from a helicopter, which required some ingenuity, as camera mounts did not exist for this purpose at the time. Using a Sikorsky S-51, UK registered G-AJOV, a former BEA airframe, the side door was removed, and the camera jerry-rigged with ropes, wires, and cables. These shots were done in July 1954, according to a "Making of" short documentary included in the DVD release.