Nick Nolte lies, forgets, gets confused and makes mistakes. He tries and fails and then he tries again. The actor admits to all of it, and there is a vulnerability about him that makes this valiant. He exists in the space between having insight into why he does what he does, and actually doing something about it. It is a space that is heavy with consequences.It is early afternoon and I am being escorted from sound stage 6 to sound stage 12. The members of the press who have been junketed in to promote Nolte's latest movie, Hulk, have been strategically manoeuvred by an army of publicists through the desert that is Universal Studios in Los Angeles.

We have been bused, fed, handed schedules and instructions. And, as in most military operations, numerous things have gone awry. The round-table interviews, which started at 7am, have run late and now it's past lunchtime; my one-on-one interview will be delayed. I am told that there's a chance I won't have the full hour. Clipboards are consulted. Cars are rescheduled. All I can think is: Nick Nolte has been up since 7am answering questions. Great.

The cavernous sound stage has a tiny door, and I trail behind the publicist across the barren space to an area that has been sectioned off with a thick black drape. Behind it is a claustrophobic science-theme setting: primary-coloured props from the film, including a yellow gamma sphere, orange and blue tubes in the shape of double helixes and glass beakers on makeshift shelves, and, in the middle of all this, a large, round table with 12 empty chairs.

A spotlight with a red scrim heats the room. Everything is fake - the lighting, the air, the atmosphere. I want to open a window but there isn't one. Eventually, footsteps approach and the drape is pulled back. Nolte appears, accompanied by a small group. He is graceful, rumpled, holding out his hand, apologising for the delay. His presence is gentle but wary. Seconds later, just the two of us, seated and facing each other. A few feet away, he exhales. In front of him, a bottle of water and a pack of Winston cigarettes.

'You mind if I smoke?' he asks. I shake my head. He lights up. 'I know it's insane - it's crazy. I've tried to quit but doing press causes me stress.'He is wearing a crumpled, lavender-coloured long-sleeved linen shirt and silky violet pants. He's into purple now, all shades of it, because it's soothing. His clothes seem lived in, like Nolte himself. His hair is a muted golden colour and hangs down over his eyes, which are blue and intense - challenging, gleaming - as he leans his 6ft 1in frame back in the chair.

Contradictions play out everywhere. His face: ravaged yet at the same time still taut and remarkably youthful. His manner is affirming - head frequently bobbing up and down with a gravel-voiced 'Yeah, yeah.' But still there is a distance. He's here, not here. There is an intelligence in his speech because he is thoughtful, but there is also an undercurrent of menacing amusement. I'm never sure whether what he is saying is real. You soon realise that with Nolte this uncertainty is the point. None of it is real. 'We're all liars,' he says.

Some of the stories he's told the press include how he left his dead father's artificial leg in a bar... How he had a testicle tuck... He lived in a Mexican brothel... He couldn't read until his mid-twenties. But fiction is a refuge, and Nolte seizes the chance - any chance - to hide.

What Nolte doesn't hide from are his mistakes. The characters he plays are often tormented, introspective men and Nolte gives their rage a powerful, physical beauty and depth, whether he's playing a Vietnam veteran in Who'll Stop the Rain, a ruthless officer from The Thin Red Line, a racist killer in Q&A, or a beaten-down son in Affliction. His characters tend to be men in crisis, and his range is staggering. The challenge, he says, is when the time comes to let the characters go.

'You have to slowly wind out of it,' he drawls. 'When you come out of it, there's depression first. Then sadness. You don't know what to do. Not quite sure who you are, or where to go. I used to try and drink my way out of it. I'd drink until the next film. That was highly destructive behaviour. The only place I ever felt comfortable in life was on stage. When I got on stage, I was home. I'm not comfortable out here. There's too much coming at me. At least on stage, I can be what I am and explore what I know. Same when I go on a film, when I walk into a trailer. Some people hate trailers - I love 'em. Because when I walk into that trailer, all time ceases to exist. All outside responsibilities fall away. I have one responsibility - and that's my commitment to the role.'He compares the feeling of everyday life falling away to having an affair. 'Everything becomes more. More sex, more holding, more touching - you become addicted because it doesn't last. It can't be sustained. It releases too many chemicals that keep this heightened state of feeling going. But you can't stay there for ever.'

Whereas most people - especially actors - are careful about what they say, Nolte's filter is missing. Some of his thoughts link together, others are seemingly coming out of nowhere, but they all, eventually, round out to make a point.

'I stopped drinking when I was 48, stayed sober 10 years and then picked it up again in 1998 - only this time,' he pauses, 'it wasn't alcohol.'

It was another substance: gamma-hydroxybutyrate (GHB), commonly known as the 'date rape' drug. It gives a euphoric effect while lowering the degree of consciousness. Nolte, who got it from scientist friends, explains that it is part of the Gaba system, the part of the brain that regulates neurotransmissions. He tells me, very matter-of-factly, that he drank it in liquid form, morning to night. 'It overstimulates, so you feel intoxicated almost immediately. You have to be careful because if you take a little too much, it will operate as an anaesthetic. Then you go into a deep REM sleep for about four hours and a lot of people don't think you're alive, but it contains a natural oxidiser, so it does keep you breathing.'

He had quit all drugs, but when he was doing some research into the genetics of addiction, he says he 'ran into' some scientists. He discovered that GHB functions as an absolute mood elevator - immediately. It was faster than alcohol. 'You stayed high for longer. There was no hangover, no side effect. I was the only person who has ever been in rehab who drank this stuff daily for four years. They didn't know what to do.'