Henry Hathaway later said he hated Glen Campbell's performance, which he described as wooden, and claimed the singer was only cast so he could have a hit with the theme song which would help promote the film.
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 U.S. release in June 1969, he 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 he should have won the Oscar instead, for his portrayal of King Henry VIII in Anne of the Thousand Days (1969).
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.
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.
While most of the characters in the film are purely fictional, there is one character who is based on a real person. Judge Parker is based on the actual Judge Isaac Parker of the Western District, who held court in Fort Smith, Arkansas, during the period of the movie, and was known as a hanging judge.
Marguerite Roberts was a formerly blacklisted writer due to her extremely left-wing politics. John Wayne, who had extremely right-wing politics, 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.
Mia Farrow, among other well-known actresses, was approached to play Mattie, but she turned it down. Robert Mitchum, whom she had just done a film with, had told her that Henry Hathaway was cantankerous and impossible to work with. She lobbied to get Hathaway replaced by Roman Polanski, who had recently worked with her successfully in Rosemary's Baby (1968), but to no avail. She later said it was one of the biggest professional mistakes of her career.
Rooster Cogburn wields a Winchester 1892 rifle with a looped lever and a Colt 1873 SAA revolver. Le Boeuf carries a Sharps single-shot rifle. Mattie uses a Colt Walker 1847 revolver. Chaney uses a Henry rifle.
John Wayne's Best Actor Oscar win was widely seen as a sentimental choice, more in recognition of his 40-year career. His performance in this movie was dismissed by many critics as over-the-top and hammy.
While John Wayne and Henry Hathaway didn't think much of Kim Darby's acting abilities, she had nothing but praise for Wayne: "He was wonderful to work with . . . ". However, she told producer Hal B. Wallis that she never wanted to work with Hathaway again.
In 1968, John Wayne was a judge on "Your All-American College Show," a syndicated talent show for young college students. Singer Karen Carpenter, along with her brother Richard Carpenter, who had not yet struck gold as the recording duo "The Carpenters," were competing on the show that week as "The Dick Carpenter Trio." Wayne was so impressed with Karen Carpenter's effervescent personality that he wanted her for the role of Mattie. However, according to Carpenter's family, the film's producers wanted to go with a name actress, and not a singer who, at the time, was virtually unknown.
Michele Carey, who co-starred with John Wayne in El Dorado (1967) a few years previously, was another actress that Wayne wanted for the role of Mattie. However, she was already under contract for another film.
The trivia item below may give away important plot points.
The scene near the end where Rooster Cogburn (John Wayne) and Ned Pepper's gang meet in a field and Pepper (Robert Duvall) is shot was filmed in a clearing near the top of Owl Creek Pass outside Ridgway, CO. The field is off the road to the left and is very easy to find.