The 53-year-old Cary Grant was only 15 years younger than Cathleen Nesbitt, who played his grandmother.
Deborah Kerr and Cary Grant improvised many of their scenes throughout filming, and a number of lines that made it to the final cut of the film came from the actors' improvisation.
During production, Cary Grant's wife, Betsy Drake, had him visiting a hypnotist to help him quit smoking. She also packed him a hamper full of health food for his lunch, though he often finished it before starting filming because without cigarettes he was hungry all the time.
The movie is set on the SS Constitution, owned by American Export Lines. Commissioned in 1951, she sailed the New York-Genoa-Naples and Gibraltar route to Europe. The sister ship to the SS Independence, they were two of the world's most famous post-WW2 ocean liners. Grace Kelly took this ship to Monaco for her wedding. "I Love Lucy" featured the ship during their trip to Europe.
Cary Grant, who had started smoking in 1911, gave up his 60-a-day cigarette habit during filming, although he continued to smoke an occasional cigarette for several more years.