In preparation for Once Upon a Time in the West, Leone cut loose from his usual writers and spent time watching classic Hollywood Westerns with the young Bernardo Bertolucci and Dario Argento. He then asked Sergio Donati (who had contributed to the scripts of his last two Westerns) to help transform their treatment – a mosaic of hallowed movie moments, held together by a simple story about water rights in the desert – into a brilliantly choreographed elegy on American myths (“Once Upon a Time”) and grown-up historical reality (“In the West”). From the opening sequences and their balletic remixes of moments from High Noon, Johnny Guitar, Shane, and The Searchers, to the climax inspired by John Ford’s The Iron Horse, this dance of death – as Leone said – collides the worn-out stereotypes of the Western with “the new, pitiless era that was advancing”. It is an anthology of great sequences from the Hollywood Western, lovingly re-created before being turned inside out. The stereotypes include Henry Fonda – cast radically against type – as a blue-eyed sadistic killer, Charles Bronson as the avenger with no name who wears a musical instrument around his neck, Jason Robards as a romantic bandit, and Claudia Cardinale as a prostitute or, in contemporary Hollywood parlance, a dance-hall girl.
Once Upon a Time in the West was filmed for Paramount between April and July 1968, with a budget of three million dollars. The opening sequence was filmed at La Calahorra, near Guadix, and, designed by Carlo Simi, the town of Flagstone was constructed next door; Sweetwater was built down the road from the El Paso set used in For a Few Dollars More, near Tabernas in Almería; and, for the first time, Leone filmed in America in Monument Valley, no less, John Ford’s great location, where Jill McBain arrives by buggy and where the climactic flashback occurs.
All of Morricone’s main musical themes were written in advance of filming and played on the set “to create rhythms”. Each main character had a theme, or leitmotif. Once Upon a Time in the West did not perform as well at the box office as Leone’s Dollars Westerns, and in America, following lukewarm previews, it was abridged by more than twenty minutes. It has since been reappraised to the point that critics now rate the movie more highly than any other Leone film. In addition, it has had a lasting impact on the post-1960s generations of Hollywood filmmakers.

Christopher Frayling, Once Upon a Time in Italy: the Westerns of Sergio Leone, Harry N. Abrams, New York 2005