During filming, the crew caught sight of what appeared to be a UFO. They filmed it and sent photos to NASA, who were impressed with the findings, but nothing further came of the discovery.
Because he was contractually obliged to provide his voice for the English version, Franco Nero was initially cast as Paco Roman. James Coburn was hired for the role of the mercenary (who was originally intended to be an American) based on his role in Flint Contra o Gênio do Mal (1966), but eventually dropped out of the role due to disagreements as to whether he or Nero would be top-billed. Coburn's role was rewritten as Polish so that Nero could portray the character with an accent. Tony Musante was cast as Paco after Nero saw him in O Incidente (1967).
The original draft of the screenplay was written by Franco Solinas and Giorgio Arlorio, and was largely inspired by Bertolt Brecht's "The Exception and the Rule." Gillo Pontecorvo was the intended director. The screenplay was however largely rewritten by Luciano Vincenzoni and several others, who re-conceived the film as a Mexican Revolution-based Spaghetti Western. Solinas and Arlorio disowned the new script, and Pontecorvo stepped down as director, believing he did not have enough experience with Westerns. He instead directed Queimada (1969), a film with a similar concept. Alberto Grimaldi then hired Sergio Corbucci to direct because of his experience with Spaghetti Westerns.
Based on his performance in the film, Franco Nero recommended Tony Musante to Giuseppe Patroni Griffi for a role in Numa Noite... Um Jantar (1969) that he and Gian Maria Volontè were unable to accept.
Jack Palance plays a character named "Curly." Palance later won an Oscar for Amigos, Sempre Amigos (1991), in which he also played a character named "Curly."