Gary Cooper was very pleased with his performance and was very disappointed that the majority of critics thought him miscast due to his age. Indeed, the film's box office failure was largely attributed to him being considered too old to play Audrey Hepburn's lover. In April 1958 he had a full facelift, but the procedure was largely unsuccessful.
The movie was a critical and commercial disaster on release. Many critics felt that the 55-year-old Gary Cooper, whose health was rapidly failing, should have realized that he was far too old for the part and turned it down, as Cary Grant did.
To dispel any impression that Audrey Hepburn and Gary Cooper actually have sex in their many afternoon meetings in his hotel room, a line was dubbed into the release print. When his back is turned to the camera in Chevalier's office, Cooper is heard to say, "I can't get to first base with her."
The Japanese newspaper article on Frank Flannagan does not mention him, but rather reads "The great newspaper king Kane dies". This is the same article from Citizen Kane (1941), with the name and picture of Frank Flannagan added in.
Cary Grant was originally offered Gary Cooper's part but turned it down because of the age difference between himself and Audrey Hepburn. Nevertheless, 6 years later Grant and Hepburn played love interests in "Charade" (1963).
The trivia item below may give away important plot points.
The original ending of the film just showed the two lovers departing together on a train, which threatened to land the film on the Catholic Legion of Decency's "Condemned List." As a result, Maurice Chevalier was called back to do the voice-over heard at the close of the film, in which he reports that the couple are "now married and serving a life sentence in New York City."