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.
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.
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.
John Wayne, aged 57, was 36 years older than Michael Anderson Jr., who played his younger brother Bud. It is estimated that Wayne's character John Elder was supposed to be in his late thirties or early forties.
According to the tombstone, Bass Elder was born in 1834 and died in 1898, making him 63 or 64 at the time of his death. The film is set in 1898 or early 1899, and his eldest son is played by 57-year-old John Wayne.
After John Wayne was diagnosed with lung cancer on 13 September 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.
Dean Martin later said of John Wayne, "Someone else would have laid around, feeling sorry for himself, for a year. But Duke, he just doesn't know how to be sick ... he's recuperating the hard way. He's two loud speaking guys in one. Me, when people see me, they sometimes say, 'Oh, there goes Perry Como.' But there's only one John Wayne, and nobody makes any mistakes about that."