"Hi, I'm Martin," he says, a little superfluously, as he extends his hand. These days the face still offers echoes of the beauty it had when its owner was a young man, still intact when he made Apocalypse Now. Time, gravity and experience have since combined to fill it out, but the 60-year-old version is still alert and expressive, and as reassuring as a president's face should be. His physique was always lean and compact, and even now that he's filled out a little, he still emits a sense of controlled, coiled strength.

It 's tempting to probe Sheen about The West Wing, and about Terrence Malick's Badlands ("that and Apocalypse are my own favourites"), Catch-22, and his work with David Cronenberg and dissident American documentarist Emile de Antonio. But we're here to talk about the movie he'll forever be most closely associated with, Francis Ford Coppola's Apocalypse Now, the restored version of which receives its British premiere at the London film festival this month.

Apocalypse Now Redux, assembled by longtime Coppola collaborator Walter Murch, offers a crystalline new dye-transfer print that reportedly had cinematographer Vittorio Storaro in tears, plus 53 minutes of previously unseen footage that, depending on your viewpoint, either deepens and enriches the original experience, or snaps its narrative spine while offering little in the way of compensatory enlightenment. We get more of Robert Duvall's Colonel Kilgore, some slapstick with surfboards, and a long sequence set on a French rubber plantation that gives some historical perspective on the war. Smaller roles - Larry Fishburne and Sam Bottoms - are slightly expanded, filling out characters and revealing a more substantial performance from the latter. The Playboy bunny sequence is extended to include an erotic interlude for Willard's boat crew, and Marlon Brando has a couple of new scenes.

The result is a fascinating flashback to the little-seen, five-hour assembly cut. It's like one of the Beatles Anthology compilations: everything awaiting the inspired subtraction or juxtaposition that will bring the finished work into perfect focus.

He's only billed third in the credits, after Brando and Duvall, but Apocalypse Now is Sheen's movie. Everything we see is from Captain Benjamin Willard's damaged perspective. Look again at the meeting with General GD Spradlin, staff officer Harrison Ford and creepy CIA spook Jerry Ziesmer ("Terminate . . . with extreme prejudice"), in which the Candide-like Willard is caught in a crossfire of baleful gazes ricocheting into his face. His eyes - huge, impossibly green - are ours, and his hard-edged voice, speaking Michael Herr's muscular narration, is our primary source of information and apprehension about what lies upriver. He's in almost every scene, and carries the main weight of the narrative; Brando and Duvall's parts are cameos by comparison.

And yet Sheen was the second choice. He got the call when Coppola belatedly realised he'd miscast Harvey Keitel. "I talked to Francis before shooting began," says Sheen. "He'd actually seen me for the part. But I'd signed to do a film in Italy in January 1976, so I forgot about it. Then I got a call towards the end of filming, asking me to meet Francis in LA, where he was getting ready to go back to the Philippines. I had a suspicion he wanted to replace Keitel, but didn't know until I got here and read the new script. He'd made a lot of adjustments. I went back to Rome for final shooting. It was holy week there and, by the Friday after Easter, I was in Manila starting work. They were six or eight weeks into the shooting , and already had their first big sequence in the can."

What greeted him in Manila, he laughs sardonically, was: "Chaos! The first location, you had to fly there by private plane. It was so remote, you couldn't get there any other way. The logistics were extraordinary - the amount of people and equipment, both very difficult to move around. You'd go down a road one day, then start back and find that the rains had washed it away. One whole sequence, the Do-Longh bridge, was filmed both with rain and without rain. We started with ponchos, rain pouring down, explosions, but didn't finish. And we came back the next night and there wasn't a drop of rain. We waited until Francis finally said, 'To hell with it, we'll film it dry!' So we film it without the rain - we've lost a whole day's shooting - and we come back the next day and, boom, it's raining again. Francis just says, 'All right, let's do it in the rain.' I'm tellin' ya!" - Sheen is still a little exasperated, 24 years later - "You wanna see all of Apocalypse Now? Watch that sequence. You had to improvise with where you were and what you had, and that had a great deal to do with what ended up on the screen."

The shoot is, of course, a cinematic legend. It was regarded as such a year before the movie was even released. Typhoons destroyed several huge sets. Sheen had a heart attack, aged only 36. Brando - obese, overpaid - was rumoured in the press to have been difficult and self- indulgent, though no one on set thought so.

"Marlon wasn't difficult at all," Sheen says. "Never. The only problem we had was the image, his presence, but he'd just dismiss it. He treated everyone the same - Francis, me, the guys on the crew. Also, out of all of us, I think he'd spent the most time in the third world. So he was more aware of the fact that the world's not made up of first-class service and over-privileged people. I was in awe, because for my generation of actors there were only two guys, Marlon and [James] Dean. And for Dean there was only one - Marlon."

And then there was Coppola himself, mortgaging half his life to pull a potential disaster out of the furnace and on to the screen. "Francis was absolutely the central force. He called the shots, he never seemed to be overwhelmed. He's such a feisty guy - a real New Yorker. He battled polio as a child, and his life has been uphill from the start. He wasn't a quitter, he was a street-fighter. All those parts of his character that gave him such tenacity came to the fore. He just would not be denied, and he accepted challenges at a personal level. He never waned - never! We almost hoped he'd throw in the towel, for our sakes. But he wouldn't. He would adjust."

One mark of the devotion Sheen and the cast felt towards Coppola and the movie can be seen in the famous opening montage of Willard's drunken mental collapse in his Saigon hotel room. "At the time, I was a drinker. Hell, I was an alcoholic. It was my 36th birthday and I'd been drinking all day. He's in a hotel room, waiting for a job. He's on his third tour, a lonely alcoholic, just been divorced - that's what all those papers are about that he's rolling around in. And also, in a sense, he's become his enemy - eating in a crouched position, adapted to the environment. Those were the elements. I was swacked - couldn't hardly stand up. Francis tried to stop me when I hit the mirror - myself, the enemy - and I said, 'No, stay away. I want this for me.' I felt I wanted to wrestle this demon. It was planned, but unplanned. When the rushes came in, I said, 'No, I never want to see that.' And I never did, until it was released."

Having spent 13 months inside the experience of the shoot, how did he feel when he saw the movie itself? "It was devastating, because the film as a whole, fully assembled, gripped me totally. I didn't see it until weeks after it opened, at the Ziegfeld theatre in NYC. I bought a ticket, went in with friends. You could smell pot inside. The size of the film was overwhelming - the scale, the ambition, and the order Francis had imposed on the footage. But the movie is also worthy, in many ways, of having these extra scenes put back in. The film only got where it got to because these sequences were shot."

Sheen is pleased that the lengthy sojourn at the French rubber plantation was reinserted. "I suspect it was pulled out because of the ending of The Deer Hunter, with everyone singing God Bless America. With the national anthem on the accordion, I think Francis was worried people might think he'd gotten it from Deer Hunter. We'd been shooting months before the Deer Hunter crew arrived in Thailand. They went home and beat us to release - by a year. So Francis adjusted - again."

After Eleanor Coppola's Notes, a written account of the film's calamitous production, the documentary Hearts of Darkness, and Peter Cowie's The Apocalypse Now Book, it's debatable how much can be profitably added to what we already know of Apocalypse Now. What one does get from Redux is a dispiriting sense of how film-makers' ambitions have atrophied in the intervening 22 years. Its production coincided with the downswing of the 1970s Hollywood renaissance. The slow-acting poison of Star Wars had already been administered to the audience, and the talents who'd made their mark in the mid-1970s (Coppola, Scorsese, Spielberg, Cimino) were all bogged down in huge productions that, apart from Apocalypse, would bomb and severely restrict their subsequent career development: New York, New York, 1941 and, especially, Heaven's Gate.

Apocalypse Now - along with NYNY - is easily the most fully realised of the quartet, and seeing it in this beautiful new version reminds us what a truly intelligent, stubborn and occasionally demented director can do with special effects, explosions, blood-bags, severed heads, loads of helicopters, a big river and, of course, a great lead actor.