John Wayne was very disappointed that his stunt double was so obvious in the final fight with Forrest Tucker.
According to a contemporary article in the Los Angeles Times, this was John Wayne's 200th starring role in a feature film. It also noted Wayne was in the "Top Ten Money-Making Stars" list for the 20th consecutive year, and for that year he was second only to Paul Newman.
Billy tells Garrett that he rode with the worst of them, including Jess Evans and Clay Allison. Allison was a notorious gunman who worked in the New Mexico area and knew Billy Bonney. Jess Evans, played by Richard Jaeckel in this movie, led a gang in opposition to Bonney's. Unlike the depiction here, Bonney didn't kill Evans, who simply disappeared after his release from prison around 1882.
Although they met some time earlier, Christopher George and Lynda Day George fell in love and later married after working on this project.