Charles Shufford, a real-life 235 pound heavyweight boxer with a 17-2 record who plays George Foreman, was given license to make his punches as real as possible, short of incapacitating the film's star.
During the opening weigh-in scene, as the shoot was running into the early morning hours, Jon Voight was taking extras cell phones and calling their homes. Stating "This is Jon Voight, don't worry, your husband is still on set working, not out in a bar, or chasing girls", sometimes leaving this message on their voice-mails.
Over a decade earlier, Will Smith (as The Fresh Prince) released the song "I Think I Can Beat Mike Tyson", a fictional and humorous account of a bout with another Heavyweight Boxing Champion. A few years later, Smith (recording under his own name) released the song "Gettin' Jiggy Wit It" which contains the line "Met Ali, he told me I'm the greatest."
As the project lay in "development hell" for more than a decade, several directors attempted to make the film until Michael Mann was finally chosen. The list included: Oliver Stone, Spike Lee, and Norman Jewison. Oliver Stone was commissioned as the project's director at one point. Stone's very first choice for the role of Muhammad Ali was Denzel Washington. However, when Washington signed on to another boxer biopic, The Hurricane (1999), Stone opted to instead make Any Given Sunday (1999). "The Hurricane" was made by Norman Jewison in the end. Spike Lee had previously directed Denzel Washington (Stone's original choice for Ali when he was in talks to direct) in the movie Malcolm X.
Chicago's Northwest Armory, at 1551 North Kedzie Avenue, doubled as both New York's Audubon Ballroom backstage and Houston's induction center. The wall to the left as Malcolm X walks down the hallway moments before his death is the opposite side of the wall on the viewer's right when seeing Ali on the drill floor refusing induction. The wall at the end of the hallway and the door through which X walks were constructed for the film, as was the trim around the supply room doors along the side. Unusual for films, the travel through the induction sequence accurately depicts the same building's exterior and interior; the cars pull up in front of the armory on the Kedzie side, the actors walk through the Kedzie foyer, and onto the drill floor, consistent with the building's actual layout. The drill floor was also featured in the video for R. Kelly's 'I Believe I Can Fly'.
Spike Lee was considered to direct this movie and even took a meeting with Will Smith but knew he wouldn't get the job when Will Smith told him he was going to need Spike to "broaden his horizons" with this movie.
The trivia item below may give away important plot points.
Shortly after Ali knocks out George Foreman, there is a close-up of Ali with what appears to be a lone white butterfly flying behind him. This is no doubt an allusion to 'Float like a butterfly, sting like a bee'.