Gian Maria Volontè and Tomas Milian frequently clashed on-set due to their political beliefs - Volontè was an outspoken communist, while Milian had left his home country (Cuba) as Fidel Castro came into power. To build on their onscreen chemistry, Sergio Sollima also intentionally provoked the two actors into engaging in boxing matches with each other.
Sergio Sollima was inspired to write and direct a film that dealt with the exchange of moral values between two characters based on his experiences in fighting with the anti-fascist resistance in World War II. He noted that during that time, he witnessed acts of courage committed by young children with minimal life experience, and acts of cowardice committed by once-brave men. Sollima later expressed that this film was a personal favorite of the films he made.
Charlie Siringo (spelt "Charley Siringo" on a poster in the film) really existed and did what the character in the film does. He worked as an agent for Pinkerton.
Gian Maria Volontè, Lidia Alfonsi and Gianni Rizzo provided their own voices for the Italian version, while the voices of Tomas Milian and William Berger are heard in the English version.
Tomas Milian has expressed that Beauregard Bennet is not one of the best characters he has portrayed, as he believes that he did not have as much material to work with as Gian Maria Volontè.