James Stewart agreed to do the film and suggested casting his friend Henry Fonda as Harley Sullivan. Fonda agreed to do it if his role was beefed up, so James Lee Barrett came up with Harley's non-stop jabbering in the opening credit sequence. It's especially ironic because in reality Henry Fonda is by far the more quiet and taciturn of the two friends.
The political subtext was a reference to the real-life politics of James Stewart and Henry Fonda.
James Stewart had planned on using Pie, the horse he had ridden in most of his Westerns, but the high altitude of the New Mexico locations proved to be too much for the animal. After filming wrapped, Henry Fonda presented Stewart with a watercolor portrait of Pie. Pie died two days later.
During filming, Stewart's stepson Ronald was killed in action in Vietnam. Although devastated, he continued with the production.