Why Woody Harrelson can't shake the legacy of Larry Flynt

Today, then, is detox day. On the table in front of him is a superfoods salad and a dark-green power shake that looks as if it's been dredged from a canal. Harrelson turned 50 last July, although the milestone didn't hit him like he expected. "It was a lot easier than I thought. It's almost like an automobile that's been chugging uphill. Now I'm on the downhill side. It's so much easier. All I've got to do is apply the brakes every once in a while."

Receding hairline apart, Harrelson – with his boyish smile, surfboard tan and blue-eyed twinkle – doesn't look his age. Perhaps because, 19 years after Cheers ended, it's still hard to disassociate him from his breakout role as the sitcom's hick bartender, Woody Boyd. After all, 200 episodes and a nine-year run: that's a lot of character to undo.

Not that Harrelson ever felt the need to run from Woody. Replaying the character in both The Simpsons and Cheers spin-off Frasier, he's not one of those actors forever whining about being taken seriously. Beginning with the 1992 slam-dunk hit White Men Can't Jump, via his pro bowler in Kingpin, right up to his recent turn as a gay GQ editor in Friends With Benefits, he's always got a kick out of making people laugh. "A comedy is just fun," he says. "

There is, however, a darker side to this man who, after Cheers, began dismantling the Woody persona with almost maniacal glee, as Mickey Knox, the serial killer at the heart of Oliver Stone's Natural Born Killers. Then, three years later, he steered himself towards his first Oscar nomination by playing the real-life porn baron who battled a landmark obscenity case in The People vs Larry Flynt.

More recently, Harrelson has played a male escort in The Walker, a paranoid junkie in A Scanner Darkly and, most potently, a soldier in Oren Moverman's The Messenger. Co-starring alongside Ben Foster, Harrelson gave a titanic turn as a military man with the unenviable job of informing the relatives of those killed in combat. It was, he says, "a terrific role".

Offering up his most intense work in years, The Messenger pushed him to the second Oscar nod of his career. So it's no surprise that Harrelson has reunited with Foster and Moverman for Rampart. Throw in the fact that the script was written by crime author James Ellroy, and it's one of the juiciest cinematic collaborations of the year – though Harrelson insisted on reading the script before accepting the part. "I don't think there's anyone I'd do 'sight unseen'," he says. "I wouldn't do Quentin Tarantino sight-unseen. You've got to read the script. But I was definitely inclined, and then I read it and I was like, 'Oh, f***! Yeah. Let's get going on this.'"

Set in Los Angeles in 1999, Rampart is, in essence, a dirty cop drama. The title refers to the specific police division where Harrelson's renegade, racist officer Dave Brown works – though it also is a reminder of the "Rampart scandal" that besmirched the LAPD in the late 1990s, after more than 70 officers were convicted of offences including unprovoked shootings, evidence-planting and narcotics-dealing. While the film leaves real-life events in the background, Brown's actions – he gets caught on camera violently beating a suspect – echo what really happened.

Yet Rampart is not just a story about police corruption. As with The Messenger, it's a portrait of a man whose very soul has been blackened by his world. Factor in his bizarre domestic arrangements – a father of two daughters by separate mothers (Anne Heche, Cynthia Nixon), who happen to be sisters, all living together under one roof – and it's little wonder that the volatile Brown is ready to implode.

According to Foster, Harrelson sucked this in. Acting with this intensity involves "giving part of yourself away", he says. "It costs something, for sure."

So did Harrelson pay a price for the role? He looks at me, nodding slowly. "The emotion that is most prevalent in this character is paranoia. And I steeped myself in a paranoia that I think was a bit costly. There were friends that I went off on. They'd be kidding about something and I took it the paranoid way. Things like that were happening quite a bit. Several of my friends said, 'I can't wait until you're done with this movie. You're not acting right.' I was short-tempered. Ultimately I think I shook it pretty well. I'm not thinking that way any more."

His mind drifts, briefly, and he offers a surprising revelation. "I was watching this show, Emmerdale, I think it's called." He recalls a "first-rate drama". (He had, at the time, been awake since 4.30am in his hotel room with jetlag.) "I'm thinking to myself, 'These guys, this is their job to be manufacturing this level of dramatic intensity every day, week after week, month after month.' That would be hard. Punishing schedule. Plus, all this emotional stuff. They were high stakes, the stuff that was going on – I was like, 'God damn, man!'"

The thought of Harrelson watching ITV's long-running soap is quite tickling; I suggest he should do a guest spot. "I don't know if that's going to happen but I did appreciate it." Still, while he might be reluctant to do a Rampart every six months – "I would not want to do that," he says – it's not unknown for him to go in too deep for a role.

Larry Flynt was "probably the movie that affected me the most", he says. "It was almost like I never shook it. I never shook that character. I still have Larry in me. I see it in so many ways. It literally changed me as a person." How? "I'd never thought I'd go to jail for something. I went to jail for s*** after that." A long-time campaigner for the legalisation of hemp and marijuana – Grass, a documentary he narrated, was shown in Westminster during the debate on declassifying cannabis – Harrelson was arrested in 1996 after symbolically planting four hemp seeds in Kentucky.

"I was going to jail for protest, which is s*** Larry would've done, but I never would've thought Woody would've ever wanted to go to jail, period," he reflects now. Ultimately, the jurors refused to convict him (despite there being video footage of the incident). "I think it took a courage that just got infused through playing Larry, you know? I found myself doing s*** Larry would do. I love the guy. I really do love him. I don't really love what he does for a living. But I love him as a person. I think he is incredibly brave."

Harrelson was born in Ohio, one of three brothers. His mother Diane was a devout Presbyterian legal secretary. But his father, Charles, was another matter. A pro gambler, he was reputedly one of the hobos arrested on the infamous Dallas grassy knoll, just after the assassination of President Kennedy. Then, in 1968 when Harrelson was seven, four years after his parents divorced, his father was jailed for the contract killing of Sam Degelia, a Texan grain dealer.

Not released until Harrelson was at college, he was later given two life sentences for allegedly being paid $300,000 to kill a judge. He died in 2007, in a maximum security Colorado prison. Harrelson has always supported his father, believing he was framed for the second murder and even donating his earnings towards a second trial. "I don't see him as a murderer," he once said. "I see him as a dad."

A hyperactive child, the young Harrelson was taunted by peers for being "learning disabled". In 2002, he hit the headlines during a stint in London's West End in the play On An Average Day when he was arrested during an incident that ended with him paying £500 towards repairs to a taxi.

Harrelson's romantic history includes relationships with Glenn Close, Carol Kane, Brooke Shields, Ally Sheedy and Frank Zappa's daughter Moon Unit. Married briefly in 1985 to playwright Neil Simon's daughter, Nancy, Harrelson has been with his second wife, ex-assistant Laura Louie, since 1987. They married three years ago, and live with their three daughters "off the grid" in Hawaii. "The one thing that is much more the 50 me, as opposed to the 30 me, is that I just love my family, and I'm psyched to be hanging with my family in Maui," he says, "whereas 20 years ago, I had wanderlust. I loved travelling. Now I don't mind just hanging out in Maui. It's a great little deal over there."

He might seem super laidback, but like most fathers Harrelson worries about his children, Deni, 18, Zoe, 15, and five-year-old Makani. "I dropped my daughter off at college recently," he says, choking up a little. "That's tough, man. I just worry - she's going out in the world, and she's just this amazing angel, y'know? And the world is tough. But it's a relatively safe thing, in that it's college and she's on a campus. But that was hard. So in that sense I worry - she's outside of my reach."

To an environmentalist, concerns for the planet's eco-system may provide plenty of grounds for any fears Harrelson may harbour for his daughters' future. "When the pilgrims came to America, we had that wonderful time Thanksgiving, where we sat down with the Indians and everything was beautiful and then extermination and genocide started," he notes. "If it was now, we would just call them terrorists. But we called them savages at the time. And instead of looking at this incredible way they had integrated nature as a part of their life, we didn't take any of that. It was, 'We need this land. We're not going to learn anything.'"

Harrelson once participated in a traditional Native American "sweat", sitting in a hut with some hot rocks, greeting each new pile with the phrase, "Hello, stone people." His spiritual life, he says, began in 1989 on a visit to the ancient Inca ruins of Machu Picchu – although it was his old Cheers co-star Ted Danson who first encouraged him to turn green, after getting him involved in the American Oceans Campaign in the United States, which lobbied against the illegal practice of drift-netting.

We could do worse than follow the Native Americans' philosophy to "look seven generations into the future", he thinks: "We're not even looking one generation into the future. It's all immediate and superficial, and how much money can we get. A fundamental change is going to have to take place, in the area of energy. We can't be getting it from nuclear power plants or oil because it's just not sustainable."

His next film, The Hunger Games, takes him into a different future. An adolescent action-adventure adapted from Suzanne Collins's novel and co-starring Winter's Bone's Jennifer Lawrence, it's set in a totalitarian society where randomly selected teenagers are forced to fight to the death. Harrelson plays Haymitch, mentor to Lawrence's character Katniss. "He's a drunken bum," he laughs. "I was doing a little research last night for the sequel!"

It's typical of Harrelson these days to take a small role in a big movie, while saving his energies for the Ramparts of this world. "Yeah," he nods. "I'm up for anything. If it's a cool part, in a cool piece, I'll try whatever. But I don't mind jumping in and saying, 'Let's jump in the ring, let's go 100% on this deal. Let's go pedal to the metal. Let's redline it'."

Personally, professionally, that just about sums him up. Harrelson, at 50, is still going full-throttle.

Rampart (15) opens on February 24. The Hunger Games (tbc) is released on March 23.