This picture marked the return of John Wayne to work after having a cancerous lung removed just four months earlier. He insisted on doing all his own stunts to show the public that the illness hadn't slowed him down.
Tommy Kirk was fired from the film after being arrested for possession of marijuana at a party on Christmas Eve 1964. Photographs of him wearing police handcuffs made the front pages of newspapers, and consequently he found it increasingly difficult to get film roles.
Despite this being a big-budget movie with a large cast, Karl Swenson was utilized to play two parts. He played "Doc Isdell" and also the bartender in the scene where the Dean Martin character auctioned his eye. The bartender part actually had more lines of dialogue.