Full bloom: Kristin Scott Thomas discovers life after the English rose

For years, she was typecast as a frosty English rose. But then something remarkable happened – and Kristin Scott Thomas blossomed into one of the most interesting actresses of our age

Kristin Scott Thomas can still remember the first review she ever read of her own work. It was in 1986, for her film debut, Under the Cherry Moon, a bizarre vanity project starring the diminutive artist formerly known (and now indeed known again) as Prince. Not long out of drama school, the then 25-year-old actress was plucked from a routine Parisian audition by the purple one to play a topless socialite. All very exciting, until the reviews came out. "It stuck in my brain for ever," she recalls now. "It said: 'Kristin Scott Thomas is a better cure for insomnia than a glass of warm milk.' " Almost a quarter of a century later, she still looks hurt by it. "It was mean, wasn't it?"

Today, we're back in Paris, the city Scott Thomas has called home for the past 30 years. While we may regard her as the most terribly English of actresses – with her cut-glass accent and her vaguely haughty air – in truth she has been adopted by the French. "Even by myself," she says. "I feel like I'm French now." It's not just that this is where she shared a flat on the Rive Gauche (along with a Burgundy farmhouse, an hour and a half south of the city) with her former husband of 17 years, the fertility doctor François Olivennes. Latterly, Scott Thomas has become a darling of French cinema, too – which may suggest why we meet at UniFrance, an annual event to promote Gallic film.

We've been allocated a room at the suitably regal Grand Hotel, in the city's historic Opéra district. When Scott Thomas arrives, she chatters away in flawless French to the PR before positioning herself in the middle of a plush red sofa. Removing her black overcoat, which she neatly places on the cushion beside her, she slides her hands under her thighs, clamping them down. If this is a sign of nerves, her businesslike appearance – long-sleeved navy dress, knee-high black boots and a wealth of gold bangles on her arms – is confident and cool. Now 50, with just the merest few lines around those subtle blue eyes, she still looks glorious: slim and elegant, in that effortless movie-star way, and with cheekbones you'd break the law for.

For years, Scott Thomas was trapped playing uptight aristocrats, beginning with the 1988 adaptation of Evelyn Waugh's A Handful of Dust and continuing through her Bafta-winning role as the brittle Fiona in Four Weddings and a Funeral. "I think that sort of stuck and it was very, very difficult for me to shake off for a long time," she sighs. "It put me off doing a lot of work in England, because I just got bored playing 'Lady this' and 'Lady that'. It all came with this other baggage." Culminating with an Oscar nod for The English Patient in 1996, it did lead Scott Thomas for a brief time to Hollywood, with roles alongside Tom Cruise (Mission: Impossible), Robert Redford (The Horse Whisperer) and Harrison Ford (Random Hearts).

But success, it seems, didn't make her happy. "I couldn't ever really say I was having a great time," she says now. "We did have a good time on Four Weddings and a Funeral. That was fun. And I had a good time on Mission: Impossible, where it was all so mad and silly. That was fun. But the other things that were so emotional, like The English Patient, it wasn't actually much fun. It was a most extraordinary experience, and I was aware of that because I was working with Anthony Minghella and Ralph Fiennes and all these extraordinary people. But it was very intense and rather painful. And I never really got to enjoy the pain." She blushes for a second. "Where am I going with this?"

In truth, it was not a film that changed things for her but a play. Appearing in the title role in a 2001 Parisian production of the Racine tragedy Bérénice, the experience "made me braver and loosen up towards things that are maybe going to hurt for a bit", she reflects. Roles began to shift as a result, initially with the thriller Tell No One, a runaway hit in which she had a small part as the lesbian best friend; that was something "no one in England would let me do," she notes. Then came a staggering turn in another French film, I've Loved You So Long, which cast her as a woman released from prison having served 15 years for killing her six-year-old son.

Arguably the finest work of her screen career, it won her a Golden Globe nomination (her second, following The English Patient). Although she lost out to Kate Winslet, and early predictions of an Oscar nod never came to pass, it was a remarkable achievement for a "tiny" French-language film to such make an impact in America. Little wonder Scott Thomas feels liberated. "French cinema has a different way of looking at women," she says. "Women of my age are still regarded as having something to say. The emotions of a woman of my age are more interesting to people who make films in France. Just because you've got a few wrinkles, it doesn't mean to say there's no life left in you."

Her latest film, Leaving, more than proves this. Written and directed by Catherine Corsini, she plays Suzanne, a mother of two married to physician Samuel (Yvan Attal), who cracks open her bourgeois existence when she has a passionate affair with an immigrant handyman (Sergi Lopez). If it sounds like a set-up we've seen a thousand times before – even Scott Thomas admits she first thought of it as "a very banal story of adultery" – the outcome is anything but predictable. Leaving her family for Lopez's working-class gruff, Suzanne finds herself at the mercy of her cuckolded husband, a man seething with jealousy, bitterness and resentment. What results is an electric performance from Scott Thomas, a sort of full-on, shackles-off turn that bristles with intensity.

"I liked this woman abandoning everything for this inappropriate relationship, when she ends up living in this foul situation," she explains. "I loved this craziness that she has. She gets swept up in the whole thing. I'm sure she's scaring him. She just loses it completely and decides to get what she wants. She says to her husband, 'I've fallen in love.' I couldn't really understand why she did that. It was a very stupid thing to do. She wants to tell him, so that he will give her permission to do that, which is pathetic! This woman is so much under this man's power that she even has to get permission from him to fall in love with somebody! It really moved me, that."

Not that it came easy. The role was emotionally and physically demanding, and her relationship with her director was tempestuous to say the least. "She shouts," sniffs the actress. "I never raise my voice!" I should point out that Scott Thomas does not see this as a bad thing. "You have to understand that Catherine is a very, very passionate and quite outrageous person. I would work with her again tomorrow. It was the most extraordinary experience." For her part, Corsini offers an interesting insight into her star. "Sergi Lopez is very easy to work with," she explains. "He's very outgoing. He's very charming and funny. Kristin Scott Thomas, on the other hand, is rather distant and cold. She's very kind but she's distant and cold."

Inevitably, problems arose around the film's intense sex scenes between Lopez and Scott Thomas. "She accepted the role," recalls Corsini. "And then we started filming and all of a sudden she said, 'Sex? Is there going to be sex? Am I going to have to do that?' And I said, 'What's the matter Kristin? You read the script before you accepted the movie. You knew you were going to be in sex scenes and you weren't going to be wearing a sweater!' So for a week I was really going crazy, wondering if she would accept it. But, in fact, she did – 300 per cent. There was a lot of apprehension, a lot of fear – and afterwards she would say, 'Did I really do that?' But she did it."

I ask Scott Thomas if it was nerve-racking to do all those sex scenes. "What?" she interrupts, "Because I'm old? Is that your question?" Her directness is quite disarming. "Er... yes!" she continues. "Because you know you're going to be judged." In truth, Scott Thomas is thoroughly convincing, helped by the fact she looks in fantastic shape. But how does she think people in England will react to such erotic scenes? "Mmm... 'Shock, horror! It's disgusting!' " she muses. Why? "Because it's a middle-aged woman having an emotional breakdown, and there's a lot of nudity. And I think people are going to be... I don't know what they're going to think. I don't know. But I'm braced."

All of this offers a fascinating insight into Scott Thomas's psychology. The archetypal actress who "you wheel in when you need a posh person", as she puts it, she evidently resents the fact that she's still defined by her class on the English side of the Channel. In truth, she was doomed from the start. Born in Cornwall, and raised in Dorset, she was educated at the elite Cheltenham Ladies College. But while she might look like she was born for public school, her seven years boarding there were miserable. "I was very unhappy at the time. It was a very difficult time for all of us lot. It was difficult to judge. I was happier at the next school I went to, which was the convent – which allowed me to live at home."

Her reference to difficulties refers to a double tragedy that struck her family. When she was just five, her father, a Fleet Air Arm pilot, died in a plane crash. Her younger sister Serena – with whom she used to yell at the skies, "Can you hear us, Daddy?" – has since said that her sibling "didn't seem to be affected by what had happened". For her part, Kristin felt, as the eldest of four, that she must stay strong for her mother's sake. "There was a lot of 'Come on now, stiff upper lip'. My mum wasn't at all like that, but she'd married into the forces and that was what she thought you were meant to do."

Re-marrying another pilot, her mother unbelievably suffered the same tragedy six years later, when her second husband also died in similar circumstances. The accident happened three days before the end of the school summer holidays, and Scott Thomas was put on a train bound for Cheltenham with "nothing" much said to comfort her. Forced to lock away such painful emotions – if you're looking for a reason why Scott Thomas may be cold and distant, as her director estimates, then look no further. The mind boggles how she ever got through The English Patient, in which her character goes down in a plane crash at one point. At the time, school provided little distraction. "It was a lot of hockey," she says. "And I was very unsporty."

At least Scott Thomas found an outlet for her feelings when, at the age of 18, she decided fully to pursue a life of acting. The film that had first incubated this desire was The Sound of Music. "I still absolutely love The Sound of Music and anything with Julie Andrews in it," she says. "I was very small, probably under ten. I knew I wanted to be an actress before that, but it somehow made things seem even better! The opening shots of her coming over the mountain – and it's all a bit shaky but you hear this music swelling up really slowly, and you see a spinning figure in this great big field. It's so fantastic!"

After school, she spent a year at London's Central School of Speech and Drama, only to be told by one particularly beastly teacher that she couldn't act, and so could not switch from the teaching course. She tried to sneak into acting classes, was spotted and dismissed from the college. Neither was she much good at making ends meet, jobs rarely lasting "for more than 10 days". A smile crosses her lips at the memory. "I was a receptionist – and got fired from that because I used to spend my time calling my friends on the phone all the time."

Living above a fish and chip shop in London, it was a bleak time for her. Depressed, disillusioned and ballooning in weight, she left for Paris – "the nearest place that was foreign" – on New Year's Day 1980 to stay with a friend. Becoming an au pair girl for a couple involved in opera, she managed to worm her way into a Parisian drama school, her English accent being her novelty calling card. It was in France that she met her husband, with whom she has three children (Hannah, now 21, Joseph, 19, and George, 9). While she frequently referred to Olivennes as her "rock", at the time of their split there was a hint of a fling, with Scott Thomas linked briefly to Tobias Menzies, an actor 14 years her junior.

More recently spotted with Steve Barron, the Irish-born director perhaps best known for his innovative videos for Michael Jackson and Dire Straits, it seems that divorce has liberated her just as much as French cinema has. As she told one interviewer, "Now I can be the mother I want to be, the woman I want to be, the actress I want to be." I wonder, now that she's divorced, whether she'd consider a return home. "I can't move back to England. My home is in France now. I'd love to but I can't. My family's all there now." She pauses for a second. "Maybe I'll retire to Eastbourne – I like the look of it. Rows and rows of little bungalows. My granny used to live quite near there."

For now, there seems little reason to retire. Having recently convinced as John Lennon's aunt in the acclaimed Nowhere Boy, a role unimaginable for her a few years back, her English-language parts are now looking up too. She's just finished The Woman in the Fifth, a drama set in Paris with Ethan Hawke, in which she plays a widow who may be involved in a series of murders. And there's also a film version of Guy de Maupassant's 19th-century novel of seduction and scandal Bel Ami, opposite the delectable Twilight star Robert Pattinson.

Rejuvenated, full of vim and vigour, Scott Thomas is flourishing. "I'm sure I'll keep going," she says. "I have a feeling I will work for a long, long time. I like it a lot... and I don't know. I just have a feeling that I'm going to be one of those people who go on for ever."

'Leaving' opens on Friday