John Emery died shortly after release in the USA.
Warren Beatty originally agreed to play the lead role but changed his mind before signing the contract.
Bobby Darin was mentioned as early choice for title role.
After Warren Beatty dropped out of title role, Stuart Whitman, George Peppard and Terence Stamp were considered for part before James Franciscus was finally cast.
Genevieve Page had requested an English diction coach for her role in this film because her accent was so thick and she feared that she would be hard to interpret. For one reason or another, she was never given one and her character thus has a French accent. To cover up this discrepancy, there is one instance where her character speaks French to someone on the phone, making it clear that the character (in the movie only) is a native French-speaker.
This film was originally going to be shot in color. However, budget cuts made in the film by Warner Brothers executives hindered its chances of being shot on color stock.
Martin Balsam:
briefly at party sequence