Stunt double Jim Burk performed the entire scene where Rooster Cogburn charged Ned Pepper's gang on horseback. John Wayne was only seen briefly in close-up, and he was riding on a trailer, not a horse.
Robert Duvall later said, "The director and I didn't get along - I don't get along with a lot of directors, but I do OK! But John Wayne was great working with. He was a good man and a very good natural actor, a lot better than a lot of people gave him credit for. He was an institution unto himself. And that final film he did, The Shootist (1976), it was wonderful what he did. So he was a good guy to work with, absolutely."
Despite its commercial success, John Wayne was not pleased with the finished film. He greatly disliked Kim Darby's performance, and while promoting the film for its US release in June 1969, told interviewers that he had starred in much better films, citing Stagecoach (1939) as an example. At the Oscar ceremony on April 9, 1970, Wayne personally told Richard Burton that he felt Burton should have won the Oscar for his portrayal of King Henry VIII in Anne of the Thousand Days (1969).
John Wayne had initially promised the role of Mattie Ross to his daughter Aissa Wayne, but director Henry Hathaway refused to cast her.
Elvis Presley was considered for the role of La Boeuf, the Texas Ranger. However, "Colonel" Tom Parker, his manager, insisted that Presley should receive top billing. The part was given to Glen Campbell instead.