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".
Filming was due to begin in October 1964, but had to be delayed until January 1965 after John Wayne was diagnosed with lung cancer.
George Kennedy recalled he was surprised to find that John Wayne still smoked cigars during filming despite having undergone major surgery, although he had stopped smoking cigarettes.
John Wayne's own stunt double Chuck Roberson stood in for George Kennedy in the scene where Wayne's John Elder character hit George Kennedy's character Curley with an axe handle.
When John Wayne is dragged into the river, a child calls out, "Come on Dad!" This was his three-year-old son Ethan Wayne, who was watching off camera and knew how ill his father was. Wayne contracted a serious cold due to filming this scene, and narrowly avoided the cold worsening into full blown pneumonia.