"He said: 'I knew I was right. I knew you wasn't pretty enough to play me,'" the 33-year-old rapper and star of "Independence Day" and "Men in Black" told United Press International.
Smith said the best endorsement he received for his performance came from Ali's daughter.
"She said it took her 20 minutes after the movie to realize she actually wasn't watching Daddy. She said 20 minutes after the film, it dawned on her that, 'Wait a minute. That was Will. That wasn't Daddy.' And that was the ultimate compliment."
Regarded for the past five years or so as one of Hollywood's handsomest leading men, Smith swore he wasn't afraid of destroying his good looks by doing all of the film's fight scenes without a stunt double.
"For this film, I was willing to sacrifice everything that had to be sacrificed to make it live up to the legacy or at least properly portray the legacy that Muhammad Ali had to live. So, if I had to hurt a little bit in the process that was fine," he said.
Thankfully, Smith was not seriously injured in any of the film's fighting scenes, but that doesn't mean he's the same man he was before he started filming "Ali."
"I am a profoundly changed man after working on this film," Smith declared. "I had experiences in Africa, experiences with the Nation of Islam, with the people who have made contact with Muhammad Ali. We were in Africa and I had the opportunity to spend some time with Nelson Mandela, and it just made me realize that, first of all, I was very happy to be making 'Ali' during this time, post-Sept. 11, to be making a film that actually means something ..."
"My experience as far as religion? It further strengthened my conviction that my relationship has to be directly with God, not through a Baptist preacher or not through any Muslim cleric or any Jewish rabbi. My relationship is directly with God ..." he said.
To play the world champ formerly known as Cassius Clay -- and member of the Nation of Islam, whose boxing license was revoked after he refused to fight in the Vietnam War -- Smith had to approach the role on several different levels.
"(Director) Michael Mann had broken down the training to become Muhammad Ali into a three-tiered course syllabus -- essentially, the first (tier) was the physical and that was the training and learning how to fight and developing my physical appearance," the popular actor and rapper explained.
"Through the physical we moved to the second tier, which was the mental and emotional. He felt that through the physical training and through becoming a fighter, I would understand more the mental and emotional space of Muhammad Ali and then there was the spiritual training. We had an Islamic studies teacher, and we basically tried to reverse-engineer Muhammad Ali's discovery of Islam," Smith said.
Asked if he thought the movie would play well in a time when American patriotism is at an all-time high, as the United States fights a war against terrorists, who claim to be followers of Islam, Smith defended Ali and called him a great patriot, pointing out that the freedom to defy the American government because of an individual's religious beliefs is one of the things that make this country great.
"Just being an American, there is a dual responsibility," said Smith.
"We have, as Americans, one responsibility, as the patriots, who fought and died in Vietnam, and then there were also the patriots on the home front, who fought against the government and stood against the war in Vietnam. Our country was established by a militia. Our American flag was originally designed as an anti-imperialistic symbol against the British government. The rules of this government are set forth in order that a Muhammad Ali would have the ability, would have the freedom of speech to be able to stand against something people could potentially see as an egregious injustice. So, I believe Muhammad Ali is the ultimate patriot, and I believe that the film is going to give off that sense of patriotism that he did his patriotic duty by standing up and saying, 'No, I don't believe in this war,'" Smith said.
He said he chose this project because he wanted to introduce Ali to a new generation, who might know the man only as a great boxer. He added that although Ali's accomplishments in the ring should be greatly appreciated, it is important that people know of his struggles with racism and injustice as well.
"His belief and his horse-blinders with his God were really simple and that was the thing I really got attached to, the real simplicity of it," Smith said.
Smith trained for nearly a year before he started filming "Ali" and gained 35 pounds for the role.
"When you take on a role like this, there were a good seven months where I lived every day as Muhammad Ali ... Some of my friends still call me 'champ.' My friends called me 'champ' for a year and a half. The process is such that you can't really take it off during the filming. You get to the point where you're not working anymore ... I was Muhammad Ali, and it actually felt weird to see Ali."
The actor went on to say that he was respectful of Ali's spirit, while trying hard not to let his performance drift into caricature.
"Billy Crystal does a brilliant impersonation (of
Ali). I wanted to do an interpretation," Smith said.
"Ali" opens on Christmas Day.