John Wayne became good friends during the shoot with Rock Hudson and even joked that he'd rather have been born with Hudson's movie star face than his own.
John Wayne liked casting himself against tall actors, so when the 6'7" James Arness became unavailable, he helped cast Rock Hudson, who was at least as tall as he. He also wanted someone to play bridge with, and knew that Hudson was a good bridge player.
Rock Hudson admitted in a 1980 interview that he thought the movie was "crap", and attributed its box office success only to the fact that it immediately followed True Grit. However, he had fond memories of the filming because he became a close friend of John Wayne and Roman Gabriel.
According to director Andrew V. McLaglen, his first choice for the role of Colonel James Langdon was James Arness, who was willing to do it but backed out just before shooting began. Rock Hudson was brought in as his replacement.
Before filming began, John Wayne had to lose most of the weight he had put on in order to play Rooster Cogburn in True Grit.
Despite his numerous anti-gay remarks in interviews over the years, John Wayne co-starred with Rock Hudson in The Undefeated, even though he knew of the actor's homosexuality. In this Civil War epic, the champion of right-wing political conservatism worked well with and even became good friends with Hudson, Hollywood's gayest (although it wasn't publicly known at the time) leading man.
During filming John Wayne fell from his horse and fractured three ribs. He couldn't work for almost two weeks. Then he tore a ligament in his shoulder and couldn't use one arm at all. The director, Andrew V. McLaglen, could only film him from an angle for the rest of the picture. His only concern throughout was not to disappoint his fans, despite being in terrible pain.
Film debut of Merlin Olsen.