Sergio Leone offered the role of Juan Miranda to Eli Wallach, but Wallach had already committed to another project. After Leone begged Wallach to play the part, he dropped out of the other project and told Leone he'd do his movie. However, the studio already had Rod Steiger signed. Leone offered no compensation to Wallach, and Wallach subsequently sued.
The chanting of "Shon shon shon" in Ennio Morricone's soundtrack was the suggestion of Carla Leone, who thought it would sound better than the original "Wah wah wah" chants. Contrary to popular belief, Morricone himself has said in interviews the chants do not represent the names of characters but are just part of the soundscape like the chants in all the other Sergio Leone westerns.
According to Sergio Leone, Sam Peckinpah agreed to direct this film after Peter Bogdanovich had turned the project down, but for financial reasons he was turned down by United Artists. Leone's collaborators (especially writers Sergio Donati and Luciano Vincenzoni), noting the director's frequent embellishment of the facts concerning his films, claim that Peckinpah did not even consider it--Donati claimed Peckinpah was "too shrewd to be produced by a fellow director".
In the flashback scenes, Irish republicans can be seen selling a paper called "Freedom", written in an Irish Celtic script. This is probably a reference to the Fenian newspaper "Saoirse", which is "Freedom" in Irish. The original "Saoirse" first appeared in November 1910 and continued as a monthly publication until December 1914, when it was suppressed by British authorities. A separate newspaper of the same name has been published by Republican Sinn Féin (a splinter group of the main party) since the 1980s.
Juan and Seán both mean "John" in Spanish and Irish respectively. When John Mallory is asked his name by Juan Miranda, he says "Seán", but retracts it, and says "John", possibly thinking the name would confuse people (it is not uncommon for Irish nationalists and republicans to use both the English and Gaelic forms of their names). It has also been speculated that "Seán" was the name of his friend from Ireland whom we see in the flashback sequences, who is otherwise not mentioned by name in the film and only referred to as "Nolan" in the screenplay.
The book that John is reading in the scene with Juan, who has a disagreement with him about the meaning of revolution, is entitled "Patriot" and is written by Mikhail Bakunin, a real-life Russian revolutionary anarchist.