This picture marked the return of John Wayne to work after having a cancerous lung and two ribs 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.
When John Wayne is dragged into the river, you can hear a child calling out, "Dad!". This was his three-year-old son Ethan Wayne, who was watching off camera and knew how ill his father was. Wayne nearly contracted pneumonia from filming this scene.
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.
When Bud tells John they could be famous like the Dalton gang, John reminds him the Daltons "were hung". Actually, three members of the Dalton gang were shot to pieces by the outraged citizens of Coffeyville, KS when the gang tried to rob both town banks simultaneously in 1892. Survivor Emmett Dalton was shot 23 times, and spent 14 years in prison.
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.
After John Wayne was diagnosed with lung cancer in August 1964, he suggested Kirk Douglas should take his part in the movie. Henry Hathaway was determined not to recast, insisting that only Wayne should play John Elder. The producers suggested Robert Mitchum or William Holden in case Wayne did not survive surgery or recover in time.