This film was originally going to be shot in color. However, budget cuts made by Warner Brothers necessitated its being shot in black-and-white.
After Warren Beatty dropped out of lead contention, Stuart Whitman, George Peppard and Terence Stamp were considered before James Franciscus was finally cast without a screen test after director Daves saw his work in The Outsider (1961) and the pilot for Mr. Novak (1963).
Geneviève Page had requested an English diction coach for her role because her accent was so thick and she feared that she would be hard to interpret. For some unexplained reason 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 is a native French-speaker.