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.
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'.
The poem Ali recites to Howard Cosell about an imaginary fight between himself and Joe Frazier is actually from a spoken-word album he recorded before his first Sonny Liston fight. The recording included the line, "The crowd never knew when they put down their money/That they'd see a total eclipse of the Sonny."
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.
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'.
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.