Filming in Paris, where he had been part of the U.S. Army when they liberated the city in 1944, was a great experience for Broderick Crawford. According to co-star David Dukes, "He could still walk into restaurants where the owner would remember him and sit him down to dinner."
Sir Laurence Olivier wanted to do his own stunts despite his infirmity. Director George Roy Hill had built for Olivier a specially designed bicycle with a motor to give the appearance that the cyclist was pedaling. Olivier wouldn't stand for this, however, and when Hill was away, he got on the real bike and rode down a hill on his own, proving his cycling competency.
Thelonious Bernard spoke very little English, and had no prior acting experience. Prior to principal photography, he spent a month at the home of director George Roy Hill, where he did a crash course in acting and English.
The first and last films which Daniel (Thelonius Bernard) watches were both directed by this film's director, George Roy Hill; Butch Cassidy et le Kid (1969) and L'Arnaque (1973) (in Venice, entitled, La Stangata).
This movie was shot in sequence.