The temperature seems to drop by 20 degrees when Mike Tyson and his minders enter the room. "Have I got to be nice to this guy?" he asks the film-maker James Toback. "No," Toback replies. "You can be as hostile as you like."

Yet Tyson doesn't seem to have the energy to muster up much hostility. He is wearing a baggy pinstripe suit that fails to disguise what's going on underneath. His belly squeezes out of his black shirt, and he can barely drag his size 15 feet along with him. His almost-beard, white flecked, is more oversight than design. His head slumps to the side as if his massive pit bull neck can't quite bear its weight. Everything is such an effort. He speaks quietly, lethargically, like a man who has been on a heavy dose of antidepressants for too long. His Maori facial tattoo, once so warrior-like, looks benign today. He could be Lennie in Of Mice And Men, the half-gentle giant who strokes the things he loves to death.

"Hello, legend," I say. Tyson looks confused, uneasy, says he doesn't take compliments well. But, for good or bad, Mike Tyson is a legend. Many experts would argue that he was the greatest heavyweight boxing champion - or at least should have been. Sure, he didn't have Muhammad Ali's wit or grace, but as a knockout puncher, none could match Iron Mike. He won his first 19 professional fights by a knockout, he was the youngest world heavyweight champion at 20, unbeaten in three years, so far ahead of the pack that there were no rivals. Then things started to go wrong.

His wife, the actor Robin Givens, went on television in 1988 alongside him and announced that he was a terrifying manic depressive and that their marriage was pure hell. In 1990 he lost his first fight to 42-1 underdog Buster Douglas. He'd become lazy and complacent, seduced by alcohol and drugs. In 1992 he was convicted of rape and deviant sexual misconduct, and served three years in jail. It should have destroyed him, and he might well argue that it did, but, amazingly, within a year of his release he regained his world title. Then, once again, he chucked it all away.

Since retiring four years ago, Tyson has done little with his life. He has boxed in a few exhibitions, put on more weight, got in trouble with the law again: in 2007, he was convicted of drink-driving after almost crashing into a police car. Three bags of cocaine were found on him, and he was given a day in jail, three years' probation and ordered into rehab. That is when Toback, an old friend, asked Tyson, now 42, if he could make a film about his life.

The result is extraordinary - pretty much a 90-minute monologue, some of it stream of consciousness. What emerges is a man who finds it impossible to censor himself. He talks vividly about growing up with a promiscuous mother who might have been a prostitute and about a father he never knew, stealing drugs from dealers as a 12-year-old, detention centre and being taken under the wing of the boxing coach Cus D'Amato, all while he was barely into his teens. Tyson is not a man who went off the rails. He was born on the skids. Somehow, and all too briefly, he managed to transcend his traumatic destiny.

We arrange to meet in the Hollywood Hills at the opulent house of another film-maker friend, Brett Ratner. There are Warhols in the loo, Bacons in the kitchen, Giacomettis on the sideboard, Toback at the centre of the conversation, but as yet no Tyson. "We could be here a while - Mike's been held up." Toback and his entourage grin at each other. It's not the first time the boxer has delayed them.

Toback is disarmingly honest about why Tyson makes such a great subject. "The movie is like the aftermath of an earthquake. It's Mike standing there amid the rubble and wondering why he has survived. Ultimately, what I feel comes through is a struggle to justify his continuing existence because the highlights of his life are gone. Usually tragedy ends in death, but here's a tragic figure who has survived. And now that I'm here, what do I do?"

Their friendship goes back 23 years. Toback, an experimental film-maker obsessed with all things sexual, had just finished making The Pick-Up Artist with Robert Downey Jr when Tyson popped into the wrap party. "He was 18, hadn't become world champion yet. He'd heard about the orgies in [American footballer] Jim Brown's house and he was like, 'Tell me about those orgies.'" Then there were the acid trips. Toback felt that young Tyson was almost too curious.

Tyson arrives a couple of hours late. Years ago, there would have been dozens in his entourage, now there are only three. One stands over me, legs splayed, eyeballing me as I talk to Tyson. It's intimidating, but also quite funny - rather than protecting Tyson, he seems to be making sure I don't escape. It's a hot winter's day in LA. We are in the garden, the sun is beating and a rivulet of sweat is running down Tyson's nose. I ask what he has learned about himself from the film.

"When I watched it alone, I realised why people had certain opinions about me. When I was upset, I got upset like everybody else, but I'm an extremist, so when I got upset, I took it to the next level. I took it to the level of being almost violently upset. And I realise, if I was sitting next to that guy, he'd make me nervous. That guy was impulsive. Unpredictable." He wants to believe - he has to believe - that is the old Tyson.

What shocked him most? "I thought I was a dick when I was crying." This is Tyson the macho man speaking, wary of losing face in front of his buddies. But that's one of the most moving moments in the film, I say - he's talking about how he was bullied as a boy. "Well, that's your opinion, of course. Only." He talks quietly, with that familiar lisp, but the answer carries a hint of menace.

As a boy, Tyson was small, fat and bespectacled, weak with asthma and alone but for the pigeons he bought with stolen money. When kids picked on him, he just ran away. One day an older bully took one of his pigeons and popped its neck in front of him. That was the first time Tyson hit out. He surprised himself because he was good at fighting, enjoyed it, found it empowering. After that, he says, people wanted to be his friend.

"I'm a good guy, I'm a good brother. There's nothing wrong with me. Just don't push me too far, you know. I'm sure everyone has a breaking point in their lives." It's hard to know whether he's addressing the old bullies or me. Tyson's speech has a hypnotic, incantatory rhythm to it.

It was D'Amato who transformed his life. After being picked up by police at 12 with $1,500 in his pockets, Tyson was sent to a detention centre, where he learned to box. On his release he was put in touch with D'Amato, a Bronx-born coach in his 70s who had discovered Rocky Marciano and Floyd Patterson. D'Amato welcomed him into his home, fed him, educated him, trained him, disciplined him, loved him. Tyson had never known anybody like this. The two became inseparable.

"Me and Cus were two megalomaniacs sitting there talking about our future, what we could do. You understand? Two guys - we didn't have anything - talking about what we could do. I imagine myself being 13, 14, watching a great fighter fight, talking about why he is a great fighter, and asking Cus, 'Cus, how could I beat that guy if I was to fight him? What would you tell me to do to beat that guy?' " D'Amato told him that becoming a champion was more a mental and spiritual discipline than a physical one.

In 1982, aged 14, Tyson went to the junior Olympics and broke any number of records, including the fastest knockout (eight seconds). D'Amato told him he needn't worry about being bullied again, and Tyson knew he was right. He chokes on his tears. "Coz I knew I would fuckin' kill them if they fucked with me."

The most important thing he learned, he says, is that he wasn't dependent on others for his survival. "I didn't need to take the handouts. It was just psychological motivation, refusing to accept what you had always accepted, refusing to accept welfare, refusing to accept being bullied any more, refusing to live your life unlawfully." As he talks, the who man minutes ago was paralysed by uncertainty radiates a frightening conviction. "I took it to extreme levels. Success is something you work hard at, you put your nose to the grindstone and you do everything you can. You're hungry, you're grinding, and you're still not guaranteed success. So I took it to another level. I said, I'm going to die to get this. I'm going to dedicate my whole life to it. Second place is not going to do it, I'm going to be champion. And being champion is not going to do it, I have to be the champion that nobody will ever forget to the end of this planet."

Millions dream of being champion. Did it feel good being one of the few who succeeded? The diffidence returns: "That's where it gets complex. It gets tricky. I think anybody can do it because I don't think much of myself. I think if I can do it, anybody can do it." The trouble is, he says, he hears so many voices in his head, and they are so often at war with each other.

I ask if he feels more pride for the great things he achieved or shame for the bad things. "I don't know. Both become irrelevant. By thinking about the bad things, I start to feel really low and depressed. When I start to think about the good things, I just get pride and egotistical. So I try to leave them both alone."

Maybe the great tragedy in Tyson's life is that by the time he became world champion, D'Amato had died. He lost his moral compass and found himself surrounded by acolytes who encouraged his excess. He bought houses by the dozen, he had more than 130 cars, he bought lavish gifts (usually cars and jewellery) for women who had sweet-talked him for a couple of minutes. At his peak, he could command $30m for a night's work, and he earned more than $300m in his career. By 2003, he was bankrupt.

Now, he worries the film might be too successful and he will end up with "too much money and pussy" again. "It's pretty dangerous. I become accustomed to it." He has either had no money or a ridiculous amount in his life, and he feels safer with none. Does he miss the drama of his old life? "No, I was addicted to drama."

In the film he calls Desiree Washington, the woman he was convicted of raping, "that wretched swine of a woman" and insists he was not guilty.

Yet he talks explicitly, often alarmingly, about his sexual preferences and how he has treated women. "I like strong women, not necessarily masculine women, say a woman who runs an organisation, I like a woman with massive confidence and then I want to dominate her sexually. I like to watch her like a tiger watches their prey after they wound them. I want her to keep her distance for at least 20-30 minutes before I devour them and take them to the point of ecstasy. I love saying no when making love. What I want is extreme. Normally what they want is not as extreme as what I want. I want to ravish them. Completely... I may have taken advantage of women before, but I never took advantage of her [Washington]."

At times Tyson paints himself as a victim - of circumstance, of liggers, of women on the make - but in the end he says he has nobody to blame but himself. I say that the strength of the film is he doesn't absolve himself: "You say you didn't do the rape, but you did some bad things to women."

"I know. The fact is, I'm not trying to win no friends. I don't want you to think I'm doing this to try to get a clean-up job, or I want people to like me. I don't care." It's true, you don't feel he's trying to pull the wool over your eyes.

Tyson shakes his troubled head. "No... sometimes my mind tells me, you think you've got these white people fooled, that they like you - you're a fucking fraud." Now he's talking with visceral intensity. "My mind is not my friend: 'You're a fraud, you're trying to fool these white people.' And I have to contain that. That's the addict talking. That's the guy who wants to get high. The guy who wants to drink the Hennessy, the guy who wants to gallivant in the street with a bunch of crude women, that's that guy talking right now. That's not you talking, Mike."

He pauses, the sweat dripping from his head. "When you go to a doctor or a psychiatrist, and they say, 'Do you hear voices?' of course we say no, because if you say, I hear voices, they go, 'Have that guy straitjacketed' and you go to hospital. But we do hear voices. Our mind does tell us things. So your mind is not your friend if you don't discipline it and control it." He tries hard now to filter his thoughts, but he worries that it's a form of lying. Thankfully, he says, he doesn't have the same intensity of feeling any more. Maybe the antidepressants have made things easier. In 2001, he told reporters, "I'm on the Zoloft to keep me from killing y'all."

When Tyson went into rehab in 2007, he admitted being addicted to cocaine and alcohol. "I'll never beat that. That's going to be a till-the-day-I-die job. That's an inside job. Nothing to do with anything else. That's just a disease I have received hereditarily."

"Simon, keep the questions to the movie," says a minder. "We don't want to talk about stuff."

"OK, I'm sorry," Tyson replies meekly, but then goes on to ignore him. "Listen, I'll talk about anything. I'm not ashamed of who I am. I understand I've got to be sold in a certain way, but I'm not ashamed of anything I've done in my life. After all, my journey, I know who I am. And I'm cool with who I am." For a second, he believes it.

But there are so many incidents in his life that he knows he can't begin to justify. On his release from prison in 1995, by now a Muslim with the name Malik Abdul Aziz and his body tattooed with images of Mao and Che Guevara, he launched the following tirade on a reporter who suggested he should be in a straitjacket. "I'll put your mother in a straitjacket, you punk-ass white boy. Come here and tell me that, and I'll fuck you in your ass, you punk white boy, you faggot... I'll eat your asshole alive, you bitch... You scared, coward, you're not man enough to fuck with me, you can't last two minutes in my world, bitch. Look at you, scared now, you ho. Scared like a little white pussy, scared of the real man. I'll fuck you till you love me, faggot." It didn't help his protestations of innocence.

After being headbutted by Evander Holyfield in 1997, he bit off part of the boxer's ear in the rematch seven months later and spat it out into the ring. Tyson was fined a maximum $3m and had his licence revoked. But boxing needed Tyson as much as he needed boxing, and a year later he was given a final opportunity. By now, though, he had lost the pace, accuracy and hunger. His sense of fair play had also gone for a burton. In 1999, he was accused of trying to break Frans Botha's arms in the ring. That same year he was sentenced to a year's imprisonment after assaulting two motorists following a traffic accident. On his release, he fought Orlin Norris and knocked him down after the bell rang. A win in 2000 over Andrzej Golota was overturned when Tyson tested positive for marijuana. His second wife, Monica Turner, the mother of two of his six children, divorced him in 2003. In his final fight, against the journeyman boxer Kevin McBride, he was a pitiful figure - slumped in a corner, legs splayed, unable or unwilling to stand himself up. Straight afterwards, Tyson announced his retirement. "I don't have the stomach for this kind of thing any more. I don't have that ferocity. I'm not an animal any more. I'm not going to disrespect the sport by losing to this calibre of fighter."

When he talks about biting Holyfield's ear or beating up boxing promoter Don King in public, for example, he simply says he was insane.

Does he think the boxing led to that type of instability? "Boxing is nothing to do with madness, it's all about control and discipline. Madness has nothing to do with it. It's what you do with the discipline, it can drive you mad, but it depends on the individual, whether they allow it to drive them mad."

Today, Tyson lives by himself in a modest house in Las Vegas. A friend, Darryl, spends a lot of time with him and manages his affairs. His great hope for the future is that he catches up with his children, and becomes the kind of father he should have been years ago. "They never had a chance to hang out with me, like all these freeloaders did. 'Dad's an awesome guy, he's a fun guy, he's a goofy guy, he likes to make people laugh, he likes to buy gifts for people and stuff' - I never experienced that with them. I've worked hard all my life to give them a great life, and I never enjoy it with them. They get to go on all these great trips to Europe, and I should be with them."

Are they seeing a different you, the goofy guy? "I don't know - they tell me that I'm funny. Ha! I don't know. I'm just glad my 11-year-old kid doesn't have to live the life I did when I was 11."

Does he box? "Oh man, no, this guy's an erudite, he's not a boxer."

And if he got into boxing? "Let him go. There's nothing more humbling. Trust me, he'd become humble." Why? "Because it's for uniquely special individuals to do that stuff. You know, you got to strike a guy, you've got to attack the guy, but you're not mad at them, they didn't say nothing bad about your mother, then you're going and your objective is to dismantle him."

Looking back, he says, perhaps the biggest problem was achieving so much so young. "If you want to see a tragedy, just take a kid who's 19, 20 years old - some kid from the hood who's got some talent - and give them $50m. I didn't know what to do. By society's standards, you reach that level and people bow down to you. I never understood that."

Is there a danger in people treating you as a god? "No, there's a danger in that I might believe it. It's not dangerous that they say it. It depends what side of the bed I wake up on, I might believe it, then it's all downhill again, and I'm in for a big crash."

Moments later he's über-man, telling me just what made him a winner: he turned apparent disadvantages (such as his height: 5ft 11in, short for a heavyweight) into pluses (surprising challengers with his upward punching); he won fights before they started by staring out the opposition. "When you look at me, you think I'm a tough guy. I'm not a tough guy. I'm a smart guy. This is not a tough guy's sport. A tough guy gets hurt in this sport. This is a thinking man's sport. You see what happens to the tough guys; you see how they start talking, you see how they start looking. Later, they become more decrepit. This is serious stuff at the highest level. This is a brutal game."

What does he think D'Amato would say to him if he saw the film and knew how his life had panned out? " 'You swear too much!' " He grins. "I never swore in front of him."

A while later, Toback calls me over, and asks me to look at the film's trailer. It's early evening, the sun is setting and the sky is a salmon pink. The trailer is book-ended by Tyson quoting Oscar Wilde's The Ballad Of Reading Gaol:

Yet each man kills the thing he loves

By each let this be heard,

Some do it with a bitter look,

Some with a flattering word,

The coward does it with a kiss,

The brave man with a sword!

Tyson says it was Toback's idea to read the poem, but he is a fan of Wilde's. "Do you know who his lover was?" he asks. "The Marquess of Queensberry's son, and you know it was the Marquess of Queensberry who invented the rules of boxing. How strange is that?"

He seems exhausted. By the afternoon, by his life, by his mind, by everything. He says he thinks it is unlikely he will ever have anything to do with boxing again. I ask why he hasn't considered television commentary. He thinks some time before answering. "I am ashamed of so many of the things I have done." In boxing or in his private life? "In the ring, too."

It's not so long ago that he told me there was nothing he was ashamed of. He smiles, and points to his head, suggesting that the last thing you should ever expect from Mike Tyson is consistency. "There's a committee going on up there." And he laughs, a little desperately. "A committee! A committee going on up there! Oh God help me!"

• The film Tyson is released on Friday 27 March 2009