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Marguerite Roberts was a formerly blacklisted writer due to her left-wing politics. John Wayne, who had right-wing political views, knew this before he read the script. He read it and liked it. He ignored people who said he shouldn't work on anything that a "blacklisted" writer wrote. According to Scott Eyman's biography of Wayne, Roberts was not herself blacklisted, but was married to the blacklisted John Sanford. Wayne wrote to her in 1969 calling her screenplay "magnificent" and hoping she might write another such screenplay with him in mind.
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.
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.
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).
The character of Rooster Cogburn was supposed to be around 40. John Wayne was 61 when the film was made. Jeff Bridges was 60 when he played Rooster Cogburn in the remake, True Grit (2010).