John Landis was one of the stunt men on this film.
Sergio Leone made hundreds of references to films that influenced him. Some were quite obvious (like three men waiting for the train as in High Noon) and some were very subtle, like the choice of Woody Strode's sawed-off Winchester rifle, similar to the weapon Steve McQueen carried in the TV series Wanted: Dead or Alive. McQueen referred to this unique weapon as a "Mare's Leg".
Sergio Leone originally offered the role of Harmonica to Clint Eastwood, but he turned it down, so Leone hired Charles Bronson instead. James Coburn was also approached for the role Harmonica, but Coburn demanded too much money.
Al Mulock, who played one of the three gunmen in the opening sequence, committed suicide by jumping from his hotel window in full costume after a day's shooting. Production manager Claudio Mancini and screenwriter Mickey Knox, who were sitting in a room in the hotel, witnessed Mulock's body pass by their window. Knox recalled in an interview that while Mancini put Mulock in his car to drive him to the hospital, director Sergio Leone said to Mancini, "Get the costume! We need the costume!" Mulock, who had appeared as the one-armed bounty hunter in Leone's "The Good, The Bad & The Ugly", was wearing the costume he wore in the movie when he made his fatal leap.
Jason Robards showed up at the set completely drunk on the first day of filming, and Leone threatened to fire him if he ever did that again. Robards was generally well-behaved thereafter, though in June 1968, after receiving word of the assassination of Robert F. Kennedy, Robards broke down and refused to perform until the day was over, and Leone decided to stop filming for the day.
Henry Fonda originally turned down a role in the picture. Director Sergio Leone flew to the United States and met with Fonda, who asked why he was wanted for the movie. Sergio replied, "Picture this: the camera shows a gunman from the waist down pulling his gun and shooting a running child. The camera pans up to the gunman's face and... it's Henry Fonda." (Until then, and with one exception, Fonda had only been cast in "good guy" roles. Leone wanted the audience to be shocked.)
Henry Fonda prepared for his role as the villain "Frank" by arriving in Italy with a pair of brown colored contact lenses and a grown mustache. When Sergio Leone saw them, he ordered them removed. Leone had planned an important close-up shot of Frank's entrance and wanted the audience to instantly recognize Fonda with those blue eyes.
Robert Ryan was offered the role of the Sheriff played by Keenan Wynn. Ryan initially accepted but backed out after being given a larger role in Sam Peckinpah's The Wild Bunch.
Ennio Morricone composed the musical score to the original screenplay by Sergio Leone and Bernardo Bertolucci. The plot was subsequently changed, and in many places, Leone directed the film to the existing musical score.
After completing the Dollars trilogy (Per un pugno di dollari, Per qualche dollaro in più, and Il buono, il brutto, il cattivo.), Sergio Leone didn't want to do another western and began working on Once Upon a Time in America. However, after the huge success of the Dollars Trilogy in the States in 1967 Leone wanted to produce films in the United States and he began selling the idea for Once Upon a Time in America, but studios wouldn't let him do it until he made another Western for them. After thinking about it, Leone concluded that he should do another trilogy which begins with C'era una volta il West, develops into Giù la testa, and ends with Once Upon a Time in America (1984). "Three historical periods which toughened America."
Although Lionel Stander's establishment is located in Monument Valley, the interiors were actually shot at Cinecitta. Cheyenne's men enter with a cloud of red dust. The red dust was actually dust imported from the Monument Valley location.
Body count: 29
Co-writer Bernardo Bertolucci says on the film's DVD that when he first suggested to director Sergio Leone that the film's central character be a woman, Leone was hesitant. Leone first budged on this subject by suggesting the introductory shot of Jill would be from below the train platform so the camera could see under Jill's dress and show she wasn't wearing any undergarments. Claudia Cardinale says she was never told this idea and says she probably wouldn't have agreed to be in the movie if it required this shot (suggesting that Leone, mercifully, gave up on the idea in the writing process).
For the opening sequence where the three dusters waited for the train, filmmakers lightly coated the face of Jack Elam with jam and began filming close-ups while letting a fly out of a jar filled with flies, attempting to get Elam's reaction as one would light on his cheek.
For this film Claudia Cardinale and Paolo Stoppa take the longest buggy ride in movie history. It begins in Spain and goes through Monument Valley.
French actor Robert Hossein, who was a good friend of Leone's, was originally to play Morton, but due to scheduling he was unable to take the part, and Gabriele Ferzetti was cast instead.
Harmonica's unfortunate brother is played by the production manager Claudio Mancini.
Leone originally intended to reunite the three stars of Il buono, il brutto, il cattivo. (Clint Eastwood, Lee Van Cleef and Eli Wallach) in cameo roles as the three gunmen waiting for Harmonica at the start of the film, but when Eastwood was unavailable the idea was scrapped.
Over half of the film's budget went to paying the actors' salaries.
The character name of "Brett McBain" was derived from two famous U.S. mystery writers, Brett Halliday and Ed McBain (Evan Hunter).