Heroes cast long shadows. Some are larger than life and become legends that grow larger still. Some find themselves on the coattails of their times dragged into greatness or further greatness. A boxer formerly known as Cassius Clay morphed into the self-proclaimed greatest heavyweight champion of all time, an icon of the Nation of Islam, then an icon of black America: Muhammad Ali.

Will Smith has yet to become a hero, though he’s made a career playing them on the big screen. But Smith has also become larger than life. He’s ridden the wave of ’80s hip-hop-flavored pop to become a star. He has become a persona of confident, smooth charm: the Fresh Prince. That persona and his rapping background would seem to make Smith a natural to play the charismatic poet-of-the-ring, Ali.

But somehow — regardless of Smith’s months of physical, mental and spiritual preparation — his performance only completely gels for brief moments before it falls apart, perhaps under Ali’s colossal weight, or is swallowed by the Greatest’s shadow. The thin Fresh Prince has been bulked up and buffed out for the role, but none of director Michael Mann’s (The Insider) camera angles can make him as monumental as Ali, who seemed the Michelangelo’s “David” of black America, his physique formed not only in training for the ring, but somehow in his struggle against the Goliath of white supremacy. Smith looks credible on the speed bag, on the heavy bag and in the ring. He could play a boxer — but not the Greatest. Smith and Mann have re-enacted some of Ali’s major bouts, but they haven’t managed to re-create them. The difference? The former may be an exacting replication, but lacks the life of the latter.

As far as the fight scenes go, Mann finds himself under the shadow of Martin Scorsese’s Oscar-nominated Raging Bull (1980), which won Robert De Niro the Best Actor Award for his portrayal of middleweight champion Jake LaMotta. Smith doesn’t seem to be the actor that De Niro was and Mann, though an excellent director, is no Scorsese. The rounds get slacker than the ropes of the ring, until Mann pushes us into the action of blurring fists.

Ali the movie is flabby and overweight, checking in at an unnecessarily long 157 minutes. Much of this time is spent in the ring, in training — and on a Malcolm X (Mario Van Peebles) subplot. Nixon writers Stephen J. Rivele and Christopher Wilkinson and The Insider collaborators Eric Roth and Mann only allude to the motivation behind Christian Cassius Clay’s conversion to become black Muslim Ali. The opening montage crosscuts a jogging Ali with shots of young Clay watching his father (Giancarlo Esposito) painting blond-haired and blue-eyed portraits of Jesus, and then fascinated by a front-page photograph of a 14-year-old black boy named Emmett Till horribly mutilated in a lynching.

In later scenes, Clay meets Malcolm X and somehow becomes Cassius X. After Ali defeats Sonny Liston and becomes the world heavyweight champion, the messenger himself, the honorable Elijah Muhammad (Albert Hall) bestows the name Muhammad Ali on the young fighter. Mann pictures Ali and Malcolm almost as casual buddies — far from icons — until Muhammad suspends Malcolm from the Nation of Islam. Ali then gives Malcolm a zombielike rebuke (“You shouldn’t have quarreled with the Honorable Elijah Muhammad.”) and Ali digresses into a Malcolm X (1992) redux, the champ receding into the background of his own biography. If Smith is no De Niro, Van Peebles is definitely no Denzel Washington. Ali’s Malcolm pales in comparison to Washington’s. He only seems to manage an ironically Christ-like pathos as he drives to the Audubon Ballroom, the place of his assassination.

Excellent performances by Jon Voight (Zoolander) and Ron Silver (Black & White) — both unquestionably believable as legendary sportscaster Howard Cosell and boxing trainer Angelo Dundee, respectively — and a breakout dramatic turn for comedian Jamie Foxx (Bait) as trainer Drew “Bundini” Brown just seem to make Ali a doughnut: There’s something missing in the middle. Mann and crew just offer us simple pieces of Ali’s relationships and almost none of the spirit of his faith. Smith rarely manages to tap into his leonine roar.

Ali spends too much time floating like a butterfly and its truest sting is that of disappointment.

E-mail James Keith La Croix at [email protected].