Before filming began, John Wayne had to lose most of the weight he had put on in order to play Rooster Cogburn in Il Grinta (1969).
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. It is unlikely Arness was seriously considered, since Wayne had fallen out with him after Arness deliberately snubbed his film "The Alamo".
According to director Andrew V. McLaglen, his first choice for the role of Col. 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. John Wayne had never forgiven Arness for failing to turn up for an interview for a part in his film La battaglia di Alamo (1960).