Naveen Andrews is full of surprises.

Take the accent. If you only know him from television sensation Lost, speaking in elegantly brushed tones as the Iraqi-born Sayid, his real-life South London twang comes as something of a shock. Not even15 years living in Los Angeles has dented it. "It's not gonna go, mate!" he says, coming on all Cockney, as he kicks back, all suited and booted, in London's Soho Hotel. Has he ever tried to lose it? "Does it sound like it?" he retorts.

Then there's the personal life. Now 44, he has two sons. The first, Jaisal, came along in 1992, several years after he began a highly unconventional relationship with Geraldine Feakins, a mathematics teacher he fell for when he was 18. After a period of drug-and-alcohol problems, he later dated actress Barbara Hershey, some 21 years older than him. During their on-off relationship, he had a second son, Naveen Joshua, from a liaison with Czech actress Elena Eustache, eventually - after a dispute - taking sole custody of the child.

The casual namedrops are also eye-opening. Right at the end of our encounter, he mentions that he's been spending time playing guitar with Goth rocker Marilyn Manson at various LA haunts, alongside ex-Sex Pistols member Steve Jones and The Cult's Billy Duffy. "He's very bright, and very funny - devastatingly funny," Andrews says of Manson. "But let's just say his schedule doesn't really suit mine. He's up at 2pm, and starts working then, and finishes recording around 5am. And I've got a kid. Maybe when I was 21 I could've done it."

As it turns out, Manson was a huge fan of Lost, with its quasi-spiritual tale about a group of air-crash survivors stranded on a desert island. "He forced me to sit and listen to his explanation of what the final episode was about." At least Manson saw the final episode, which is more than can be said for Andrews, who seems almost off-hand about his six seasons on the show. "I just saw the pilot. I've never seen an episode. Obviously you've read the bleeding script; you know what's going down. It doesn't mean you have to watch it!"

Based in Hawaii during this time, while he remains close to some cast members such as Maggie Grace, Andrews clearly found the experience frustrating on some level. "We were all restricted by the form; it was a very set form, which became apparent after the first season." Inevitably, being tied to the show meant his moviemaking chances were restricted - though he did make appearances in Robert Rodriguez's sci-fi/horror Planet Terror and Neil Jordan's urban thriller The Brave One.

Yet nothing he's done in film compares to his latest effort, Diana. Already one of the most speculated upon movies of the year, this biographical account of the last two years in the life of Princess Diana deals specifically with her relationship with Pakistani heart surgeon Dr Hasnat Khan. Playing Khan to Naomi Watts's Diana, Andrews was attracted to the film's core relationship, he says. "It was an intimate love story, which was very rare. It reminded me of the David Lean film Brief Encounter in a way, in its quietness and its intimacy, and you just don't see that in the cinema anymore."

The film is directed by Oliver Hirschbiegel - "an artist" according to Andrews - who has form in this area. In 2004, he made Downfall, about the last days of Adolf Hitler. When I ask Andrews if he expects Diana be controversial, he simply shrugs. "Not really. It's very respectful to the parties concerned; for one very iconic figure and someone less known to the public but very honourable and decent human being, I think it's very respectful to both of them."

Based on Kate Snell's 2001 book, Diana: Her Last Love, the film has already been condemned by the real Khan, who for the past 16 years had maintained a dignified silence over his time with Diana. But in a recent rare interview, Khan told The Mail On Sunday that he has "never given any approval" to the film. "It's based on gossip and Diana's friends talking about a relationship that they didn't know much about… it is all based on hypotheses and gossip."

The real Khan has not been the only critic of the film. Early reviews have been brutal, suggesting it veers wildly in tone from a dark brooding romantic drama to a rom-com, as the Queen of Hearts is romanced by a man who operates on them. Scenes of her washing up in his flat and even impersonating a Liverpool accent on the phone to be put through to his office don't sit well - although at least Andrews has escaped the bile (The Independent dubbed him "personable and charming").

Naturally, he defends the portrait. "Reading the script, it gave me compassion for Diana, which I'd never really felt. My whole attitude to the royals was obviously influenced by The Sex Pistols, and all that kind of thing, and you didn't really see them as human beings, but an institution. Of course she was outside that institution, but I didn't really realise to what extent. So I had a whole new empathy for her as an ordinary human being who was very lonely and just wanted to have a life, like everybody else."

Curiously, it marks a return to British film - albeit with a German director - where Andrews first began his career, in the 1991 Hanif Kureishi effort London Kills Me. "Hanif gave me my first job," he smiles. "I was straight out of drama school."

Raised in Wandsworth, the son of a psychologist and a businessman, both immigrants from Kerala, India, Andrews always knew he would "not do what was expected" and acting seemed like a natural fit. "What was the alternative - 9 to 5, like my mum and dad? That was never going to happen."

Leaving school with no qualifications when he was 15, his parents "had no choice" when it came to his headstrong behaviour. "Poor souls didn't really have a lot of say in it. They've both passed so wherever they are now I hope they're relieved." Auditioning for London's Guildhall School of Music and Drama, a peer of Ewan McGregor's, Andrews flourished, although it was his small role in Kureishi's film that would prove far more of an education.

From there, he was cast in the title role of the TV mini-series adaptation of Kureishi's novel The Buddha Of Suburbia, playing a 17-year-old dealing with his cultural heritage. "He wrote a wonderful part for me," Andrews nods. "I saw it again recently. I thought it's safe to watch it again now - it's been 20 years. I look at that, and I'm very proud of it." Yet there was an even bigger surprise. "I can look at that and see similarities with my eldest son: certain physical gestures."

While Andrews would go on to appear in The English Patient, when I ask if he felt like he was a part of the British film industry, he looks at me rather blankly. "Well, I was just doing what I was doing. I didn't think of it in those terms… I was just working. I didn't really have a concept of the industry at the time. I don't think I had a concept for living, to be honest, back then! Let's just say when I was younger, I was out to lunch a lot of the time. It was a miracle anything got done."

In the past, Andrews has been upfront about his alcoholism and a two-year period in the mid-1990s when he was using heroin. It was only when he went to LA - to move in with Hershey - that he got sober. Did talking about them publicly prove cathartic, in that his experiences might help others? "I didn't think of it from that point of view at all. If it has helped anyone, I'd be very happy. But I think I was just expressing relief that I had found a way of functioning without those things."

These days, he's a "full-time parent" to his younger son, Naveen, now 7, "and that's really hard work - in a good way. There's never been any difficult age with him. And there wasn't with my eldest either. He's been great too. Even when he turned 16, 17 - around the time I left home - he was a bleeding saint!" He seems amazed by this, as if fazed by one of those 'How did I get here?' moments. "It's not 'How did I get here?'" he says. "I'm just grateful to be here."

Diana opens in cinemas next Friday.