Joel McCrea was originally cast as Westrum and Randolph Scott was Judd. But early in the production each actor went to the producer on his own, dissatisfied and ready to quit, so the roles were reversed.
Contains the quintessential scene of a cowboy riding hell-bent-for-leather toward the camera, firing his Colt revolver as he comes. Each shot he fires creates a large cloud of gunsmoke because of the historically correct black powder in the cartridges, and one such cloud completely obscures him until, a second later, he rides right through it and into view again.
According to David Weddle's book on Peckinpah, "If They Move, Kill 'Em!', four days into shooting, a snowstorm on the original Inyo National Forest location forced the entire cast and crew back to Los Angeles to resume shooting the film in the Santa Monica Mountains, which resulted in the soapsuds substitution for real Sierra Nevada snow in the scenes at the Coarsegold mining camp. The substitution not surprisingly irritated Peckinpah immensely, but he pressed on. Despite these problems, the film finished a mere four days over schedule, and only $52,000 over budget.
The trivia item below may give away important plot points.
In the original script, Randolph Scott's character doesn't survive the climatic shoot-out but Joel McCrea's does. During Sam Peckinpah's rewrite, he felt it more poignant that the Gil Westrum character (played by Scott) is redeemed by promising the dying Judd that he will deliver the gold, so the characters' outcome were reversed.