Trivia

James Stewart agreed to play a cameo role in the film only because John Wayne had specifically requested him. His brief screen time proved to be rather difficult. The bad acoustics of the huge, hollow sound stages worsened his hearing difficulties, and he stayed by himself most of the time. He and Wayne muffed their lines so often in the main scene between them that director Don Siegel accused them of not trying hard enough. Wayne's reply was a variation on an old John Ford line, advising the director, "If you'd like the scene done better, you'd better get a couple of better actors." Later on, the star told friends that Stewart had known his lines, but hadn't been able to hear his cues, and that in turn had caused his own fumbling.

When J.B. Books (John Wayne) arrives at Dr. E.W. Hostetler's (James Stewart) office, Hostetler mentions that it has been 15 years since they last saw each other. The inside joke is that Wayne and Stewart last worked together on The Man Who Shot Liberty Valance (1962), 15 years before. The undertaker, John Carradine, also appeared with Wayne and Stewart in that film.

John Wayne fell ill with influenza during production and was hospitalized for two weeks, during which time production was shut down. It was uncertain at one point whether the film would actually be completed because Wayne was so ill his doctors were close to forbidding him to finish it.

James Stewart had not made a film for five years. He agreed to play the doctor as a favor to John Wayne.

Lauren Bacall, James Stewart, Richard Boone and John Carradine were all cast at John Wayne's request.