Comparing this movie to documented history shows many factual errors. Karen Black's character of Ernestina Crawford, sister to Emmet Crawford in the movie, was completely fictitious. General Crook appears in the movie after the point he historically died.
William Goldman had come across the story of Tom Horn while researching that of Butch Cassidy and became fascinated with him, calling Horn "the most talented man who ever lived" in the Wild West, deciding to write a screenplay about him. In 1974 it was announced Goldman, Robert Redford and Sydney Pollack had formed a company to make the movie. Goldman was reportedly paid $500,000 for his work.
In 1977 Mike Medavoy at United Artists announced he had bought William Goldman's script to star Redford. Pollack was still attached and he got David Rayfield to rewrite the script.
However the project did not proceed as a feature and wound up being done for TV by CBS.
William Goldman later admitted he never saw the mini series. "Couldn't bring myself to... this was one of my bad experiences."