The Big Picture: Ali (15)

The subject is irresistible: Muhammad Ali, not only the most celebrated boxer of the 20th century, but a showman, a rebel, and an avatar of black consciousness and single-minded probity during a critical era of American history. In Ali the writer-director Michael Mann works hard to render a convincing portrait of this multi-dimensional sportsman, and at times the picture thrums with real excitement – the extended fight sequences look (and sound) amazingly lifelike, while the snap and crackle of certain performances keep things on the boil for most of its two-and-a-half-hour running time.

That's the good news. But while Ali certainly throws some nifty combinations, it must be conceded that it's far from a knock-out. "Forget what you think you know" runs its curious tagline, the problem being that the film rehearses an awful lot of what we do know and rather skimps on what we'd like to know. Could this be to do with the script? No fewer than five names jostle for the credit – Mann and three co-writers (Stephen J Rivele, Christopher Wilkinson and Eric Roth) worked from a story by Gregory Allen Howard – which on a biopic feels somehow less reassuring than a coherent single vision.

Most of their energy has gone into filling out Ali the public icon, the fighter who wouldn't quit, the motormouth they couldn't quiet, leaving a noticeable hole where Ali the private man should be.

Much depends on the performance of Will Smith in the title role, and I very much liked the way Mann waits a full quarter of an hour before a single word comes out of Ali's mouth. This little prologue exhibits Mann's film-making at its most compressed and elegant, as Ali – the young Cassius Clay, as then – prepares for his 1964 title fight against Sonny Liston, running on dark city streets, working the punchball, and remembering moments of a Kentucky boyhood – sitting at the back of the bus, seeing a newspaper photograph of a lynched black man. He intercuts this with a live club performance of Sam Cooke (David Elliott), another great entertainer at his incandescent peak, though Cooke, unlike Ali, would not live to see the end of the year.

Once Ali does start talking he never looks likely to stop. "I must be the greatest," he declares, as if it were a philosophical necessity rather than a boastful taunt. Smith has caught the musical lilt of Ali's voice, and has bulked up his body to fighting weight; he learnt to box in the Ali style, too, skipping and swaying out of the reach of a punch, and the body blows we see him take in the ring are landed by actual fighters impersonating his opponents. Smith has plainly put in a tremendous effort, though he never quite disappears into the role; he's still the Fresh Prince, not the King.

This isn't the overmastering problem with Ali, however. Mann strings his narrative between the two poles of 1964, when he beat Liston, and 1974, when he won the title from George Foreman in Zaire. The years in between are Ali's calvary, when he joined the Nation of Islam, refused induction to the Army and made himself notorious with the comment, "I ain't got no quarrel with them Vietcong." Momentous events, these, and one looks to the film to explain, or at least suggest, why Ali chose to act as he did.

But the film fails to do so. Ali's decisions come across not as the result of religious or political conviction but as pure stubbornness. Even when the authorities were prepared to cut him a deal whereby he could continue boxing while paying lip service to the army, Ali still refused. In the event the Boxing Commission stripped him of his title, and he was ostracised by the black Muslims, too.

At the time this must have been explosive and fascinating news, yet Mann and his writers, working entirely from the outside, present no plausible psychology of the man. How seriously, for instance, did he take his allegiance to the Nation of Islam? At one point we see him snubbing his friend and religious mentor Malcolm X for quarrelling with their leader. Yet he blithely ignored a cornerstone of its racist credo (the inferiority of the white) by employing and befriending white men; much is made of his affectionate verbal sparring with sports commentator Howard Cosell (Jon Voight, unrecognisable beneath toupee and make-up). His marriages and romantic entanglements are similarly sketchy.

His first wife Sonji Roi (Jada Pinkett Smith) is banished from the Ali court for apparently not dressing properly; later, hints are dropped that Ali is a philanderer, but there's nothing too shabby to tarnish his crown. Mann's film feels strangely becalmed during these tumultuous years, and because of its reluctance to pay Ali the honour of suspicion he remains a stoic, almost distant figure. The man was brave, no question, but was he never doubtful, never afraid? This account shuns complexity at every turn. The fact that Muhammad Ali himself was a consultant on the film ("This is the only film that will tell it like it is," he says) suggests that Mann, Smith and co may have pulled some punches to keep their hero onside. Their reverence actually argues that the film does anything but "tell it like it is".

The best part of Ali is its final quarter when the scene shifts to Zaire, 1974, and the gladiatorial Rumble in the Jungle with George Foreman. Smith assumes a properly imperial insolence, and gets to do his ace Foreman-as-The-Mummy impersonation at a press conference. Mykelti Williamson contributes a fantastic cameo as ringmaster extraordinaire Don King, briefly wrongfooting Ali himself with a display of bravura bullshit ("Be Moses in reverse – do not let my people go!').

The fight, edited so swiftly it sometimes becomes a blur of fists, is marvellously staged, yet tells us nothing we didn't already know from Leon Gast's 1997 documentary When We Were Kings and William Klein's recently released The Greatest, and in terms of energy and I-was-there astonishment none of them comes close to Norman Mailer's prose epic, The Fight: "Ali finally came off the ropes and in the last 30 seconds of the round threw his own punches, 20 at least. Almost all hit – one punch turned Foreman's head through 90 degrees, a right cross of glove and forearm that slammed into the side of the jaw; double contact had to be felt; once from the glove, then from the bare arm, stunning and jarring. Walls must begin to crack inside the brain. Foreman staggered and lurched and glared at Ali and got hit again, zing-bing! two more."

That's just what Ali needs: a bit more zing-bing.