After award-winning composer Leonard Rosenman recorded a score for the film, which he personally didn't care for but was given freedom to be experimentally creative, the score was rejected. While Jerry Goldsmith is credited with "Music" on the film's credits, the credit is misleading as he composed no original score for the film, instead it was tracked with cues from four other films he scored: 100 Rifles (1969); Rio Conchos (1964); Morituri (1965) and Stagecoach (1966) . Which is why he did not receive a credit like "Original Music composed & Conducted by".
In his book "The Actor's Life" (1979), Charlton Heston says that Sean Connery and Robert Shaw were considered for the role of Zach Provo, and that Jack Smight and possibly Stuart Rosenberg were considered for the director. In the end, James Coburn played the part of Provo, and Andrew V. McLaglen, the veteran director of Westerns, stepped into the director's chair.
Brian Garfield did an uncredited rewrite on the script during production. The script is very faithful to his source novel.
This motion picture is set in 1909 Arizona, which is arguably right at the end of the old wild American western frontier era. If not, it already ended by the 1890s.
Reportedly, source novelist Brian Garfield liked this screen adaptation of his "Gun Down" novel, and gave the film a "thumbs up".