Will Smith actually turned down the role of Ali. He relented after Ali called Smith and asked him to take the role, because he was the only one as good looking as him that could play the role.
Will Smith and director Michael Mann offered to put up their salaries in case the film went over-budget.
During the opening weigh-in scene, as the shoot was running into the early morning hours, Jon Voight took extras cell phones and called their homes. Some people got voice mails saying "This is Jon Voight. Don't worry, your husband is still on set working, not out in a bar, or chasing girls."
To get in shape, Will Smith moved to Aspen, Colorado where the air is thinner. He would get up at 6:30 a.m. to go for five-mile runs before hitting the gym every day for three hours of ring work and a full afternoon of weight training. The actor put on 35 pounds of muscle in less than four months.