Spaghetti The Good, the Bad and the Ugly, the Dollars trilogy changed the face of Westerns forever and made Clint Eastwood a star.

RELATED: Paolo Sorrentino & 9 Other Great Italian Directors Every Film Lover Should Check Out

But Leone made another Western masterpiece - 1968's Once Upon a Time in the West. The movie failed to make a splash in the United States and was considered a box office disappointment - especially after the wild success of The Good, the Bad, and the Ugly. But over time, Once Upon a Time in the West has grown in stature and is now regarded as (arguably) the finest Western ever made.

Leone Was Retiring From Westerns

The young cast of Once Upon a Time in America standing in old style costumes

Despite being widely known for his Westerns, Sergio Leone was retiring from the genre. He did all he wanted to do with the genre, and he was hoping to move on to other material. His ion project would eventually become starring Robert De Niro, James Woods, and Joe Pesci.

However, it would be decades before this vision materialized (the movie wouldn't release until 1984). Paramount had a lot of convincing to do. Luckily, they had just the thing ...

Leone Made The Movie Because He Wanted To Work With Henry Fonda

Henry Fonda as Frank wearing a black hat in Once Upon A Time In The West

... and that thing was beloved American actor, Henry Fonda. 12 Angry Men (Best Picture, having co-produced the film with Reginald Rose). 

Sergio Leone was a huge fan of Fonda and had long wanted to work with him, but was never given the opportunity. Paramount offered Leone Fonda and a hefty budget, and Leone couldn't say no.

Fonda Was Convinced To Do The Film By Eli Wallach

Tuco glowers up at Blondie in The Good the Bad and the Ugly

Frank is now one of Henry Fonda's most iconic roles. That's for two primary reasons. The first is that the performance is obviously excellent. But the second is because Fonda was widely known for his heroic roles, and Once Upon a Time in the West subverted that idea by making him the villain.

Despite Leone's enthusiasm, Fonda was hesitant in taking the role. It wasn't until he consulted with Eli Wallach (who played Tuco in The Good, the Bad and the Ugly) that Fonda accepted, as Wallach had sung the praises of Leone.

Harmonica Was Originally Offered To Clint Eastwood

The good, the bad, and the ugly - Clint Eastwood

Despite the newfound maturity and somber tone, Once Upon a Time in the West was a Sergio Leone Western, and fans were expecting Clint Eastwood. Who they got instead was Charles Bronson. Eastwood had been working with Leone for years, starring as The Man with No Name in his iconic Dollars trilogy.

RELATED: 10 Best Performances In Modern Westerns

Leone had offered the role of Harmonica to Eastwood, but Eastwood turned it down, wishing to avoid being typecast as "the silent Western guy."

Henry Fonda Wanted Frank To Have A Different Look

Henry Fonda and his signature blue eyes

Perhaps to distance himself from the villainous role of Frank, or maybe to help viewers disassociate "Henry Fonda" from Frank, Fonda tried having a different look. For his first day on set, Fonda appeared sporting a beard and wearing brown lenses.

Leone was incensed, as he wanted the near-mythical "Henry Fonda" and to provide 1960s audiences with the shock of seeing Fonda as a villain. He forced Fonda to shave and take out the s, so Frank appears in the movie clean-shaven and with Fonda's signature blue eyes.

Actor Mickey Knox Helped Write The Dialogue

Charles Bronson in Once Upon a Time in the West

Fans of Leone's Dollars trilogy will likely take note of the dialogue found in Once Upon a Time in the West. Whereas those three movies were peppered with unassuming, borderline comical dialogue, Once Upon a Time in the West was far more poetic, offering beautiful lines of dialogue with undercurrents of metaphor and mythical grandeur.

That's because actor Mickey Knox served as the movie's English translator/adapter, and he helped Leone pen some of the movie's more iconic and poetic lines of dialogue.

Most Of The Movie Was Shot In Spain

The opening scene of Once Upon a Time in the West

It's amazing how much Spain es for the American West. Like many of Leone's older Westerns, a good chunk of Once Upon a Time in the West was shot throughout Spain. These sequences include the opening train station shootout, the scenes involving Flagstone, and Sweetwater Ranch.

RELATED: Top 10 Westerns Of The 21st Century According To IMDb

The exceptions include all the interior shots (which were filmed on a soundstage in Rome) and the scenes taking place in Monument Valley.

The Score Was Completed Before The Movie

A closeup of Charles Bronson in Once Upon A Time In The West

In an odd and unconventional reversal from the norm, Ennio Morricone's iconic score was actually completed before the movie itself. Typically, music is scored in post-production, with the composer working directly with the finished footage.

In this case, the music was the first thing completed. That's because Leone wanted to play the music during filming in the hopes of inspiring performance, mood, and atmosphere.

Leone Consulted Many Classic Westerns

Jeffrey Hunter and John Wayne sit atop horses in The Searchers

Leone's intent with Once Upon a Time in the West was to turn the Western genre on its head and subvert most of its tropes. To conduct research, Leone and his creative consulted dozens of "classic" Western films, and the movie contains many overt references.

For example, the McBain funeral is taken directly from the 1953 classic, Shane. The ending, and the massacre at the McBain farm, mirror John Wayne's The Searchers. The auction scene and Cheyenne's dusters were meant to reference The Man Who Shot Liberty Valance. Those are but a few of the dozens of references.

One Of The Three Cowboys Ended His Life In Costume

Three cowboys stare at Harmonica across train tracks

Unfortunately, the making of Once Upon a Time in the West comes with a tragic story. Actor Al Mulock stars in the movie's opening train station sequence, playing one of Frank's hitmen. Shortly after filming his scenes, Mulock jumped out of his hotel window wearing his cowboy costume. He survived the fall itself but died in the ambulance on the way to the hospital.

The reasons for taking his own life are unknown. However, Mickey Knox (the English script translator) claimed in his book, The Good, the Bad and the Dolce Vita, that Mulock was a heavy drug and was unable to acquire drugs while filming in Spain, resulting in his death by suicide out of despair and torment.

NEXT: Steve McQueen's 10 Best Westerns, Ranked According To IMDb