Kristin Scott Thomas has had enough. She has been working flat out for 30 years, and made 65 films: having lived in Paris since the age of 19, she effectively has two careers, one in English and one in French. Even her cleaner thinks her workload is insane. Last September, for the first time in her life, she reached a point where she could no longer go on. "I just suddenly thought, I cannot cope with another film," she says. "I realised I've done the things I know how to do so many times in different languages, and I just suddenly thought, I can't do it any more. I'm bored by it. So I'm stopping." Having always been a workaholic, she now likes to think of herself, she jokes, as a "recovering actress".  For the past 15 years, the actor has fashioned a highly respected career out of small parts in biggish English-language films, such as the John Lennon biopic Nowhere Boy, and big parts in smaller French films, such as the prisoner returning home in I've Loved You So Long. But she is tired of working on films whose scripts keep being rewritten at the last minute. "The kinds of films that I do are usually quite rapidly put together, and it always seems to be a little bit of a shambles. I like filming, but what I don't like is having to rearrange things and rewrite scenes. I just can't be bothered."  She is tired, too, of being cast in films that need her more than she needs them. "I'm often asked to do something because I'm going to be a sort of weight to their otherwise flimsy production. They need me for production purposes, basically. So they give me a little role in something where they know I'm going to be able to turn up, know what to do, cry in the right place. I shouldn't bite the hand that feeds, but I keep doing these things for other people, and last year I just decided life's too short. I don't want to do it any more."  This is the second time Scott Thomas has decided to walk away from work that many actors would kill for. The first was in the late 1990s, when the success of Four Weddings And A Funeral had catapulted her into Hollywood A-list roles in The Horse Whisperer, Mission: Impossible and The English Patient. "But it was geographically unfriendly, because I just didn't want to go to America." She was married to a Frenchman, had two young children, and the family wanted to live in France or, at the very least, Europe.  But it wasn't just about that. "No, it's also that I can't bear all the kind of rubbish that goes on on those big films. I just can't stand sitting around for hours in a great big luxury trailer, waiting, bored out of my head. I used to do," she offers drily, "a lot of tapestry. Yes, I had a lot of cushions around."  She did have one minor relapse five years ago when she appeared in the airhead comedy Confessions Of A Shopaholic, but at its mention her face freezes in a pantomime of horror. "I thought it would be quite good fun. But I spent my entire time waiting. I hated it, hated it, hated it, and I said that I wouldn't do another one." Grinning, she adds, "Funnily enough, I haven't been asked to."  Actors are seldom this candid. It is an unwritten rule of the profession to speak highly of every film you've ever been involved in – but then, Scott Thomas has always been a singular sort of film star. Very few British actors work in a foreign language, for a start, and when she arrives at the studio in Paris where we meet, she's so out of the habit of speaking English that she keeps beginning sentences in French. She is makeup-free and wearing glasses which, she says, are very useful for deflecting attention: "Because people don't think, oh, who's that old woman with the round glasses?"  There is no publicist in attendance, and she submits to the attentions of the hairstylist and makeup artist without fuss. She has the sort of informality that comes from having done this many times before, and talks freely without appearing to wonder or worry about how she might sound in print. At 53, she is more preoccupied by how she looks.  Scott Thomas is mesmerisingly beautiful, but she refers to herself as an "ageing actress" and keeps saying, "I'm terribly old" or, "I'm positively ancient." In fact, being ancient might make things slightly easier, for she adds, "I'm sort of, as the French would say, 'stuck between two chairs', because I'm no longer 40 and sort of a seductress, and I'm not yet a granny." She got a terrible shock a few years ago when a close-up portrait taken for this newspaper was reprinted in a glossy magazine, without being digitally retouched. "It was a really great picture, and looked fine on a grainy piece of paper in black-and-white," she says now. "But on the gloss of a magazine cover, without being retouched, it was the most terrifying thing. It really brought home to me how the images of women we see are so manipulated." Accustomed these days to seeing faces digitally perfected, anyone who saw the magazine would, she groans, have thought: "My God, Kristin Scott Thomas, she's so ancient!"  As a middle-aged woman, she finds herself increasingly overlooked in everyday life, and has talked a lot in recent years about the unnerving experience of becoming invisible. So it's ironic that her latest film is called The Invisible Woman (the title actually refers to its young heroine, Charles Dickens' secret mistress, Nelly Ternan). It is a screen adaptation of Claire Tomalin's biography of the 18-year-old actress, who captured Dickens' heart when he was a married father of nine and more than twice her age. Felicity Jones plays Ternan and Scott Thomas plays her widowed mother Catherine, who oversees the affair with worldly maternal anxiety, part Victorian chaperone and part pragmatic pimp.  It is a slow-paced but beautifully shot and absorbing film. For Scott Thomas, its principal appeal was the chance to work again with Ralph Fiennes, who plays Dickens and also directed the film. Like her, he puts in an electrifying performance – which is all the more remarkable given that he appears on screen in almost every scene and has previously directed just one film. "It's a brilliant performance," she says, "and it was so lovely to work with him again. But it wasn't just that. There was something about playing a woman who has to make decisions about her child and evaluate her child's chances, and I found that very moving, the responsibility of the mother of this daughter. I just wanted to do it."  The only downsides were the corsets and petticoats and bustles. Her waist had to be drawn so tightly that she was physically sick, while the petticoats were so heavy that her arms would ache just to lift them. "I didn't want to wear the wigs and I didn't want to wear the costumes. I don't know how those women did it," she marvels coolly.  Wardrobe discomfort notwithstanding, however, period dramas often seem to provide the most substantial roles for older actresses, raising the disquieting thought that perhaps older women were accorded greater status in Victorian times than they are today. "I don't know," she ponders. "It's an interesting thought. Is it to do with our mania for youth, our kind of addiction to youth?" Then again, she points out, The Invisible Woman is a tale of a 45-year-old man leaving his wife of 22 years in 1858 for a woman not even half his age, so perhaps then and now are not so different after all. If anything, the parallels between the two eras are "more or less literally dramatised by the film – by the fact that Ray is getting to jump into bed with the lovely Felicity Jones". In The English Patient, Fiennes and Scott Thomas played lovers; 18 years later, the 51-year-old actor's love interest is an actress 21 years his junior, while Scott Thomas must play her mother.  I tell her that, for me, the film's most compelling characters are all middle-aged. Alongside Fiennes she co-stars with the magnificent Joanna Scanlan and Tom Hollander, and it's they who bring the screen alive. "Well," she nods, "I don't want to go and see a film about young people. I'm just not remotely interested. When I go to the movies, I'd rather watch people who've lived, who have gone through the mill, who've had their heart broken a million times and are still looking for love. That's what's interesting to me."  She thinks the film industry might just be starting to realise there is a market for films about older people. The population is ageing, for a start – "So they've got to make something to entertain us" – and films now have a commercial lifespan far beyond box-office receipts, making teenagers' dominance of cinema seats less important. "I just think there really need to be stories about people who have been through life and are still hopeful."  She is encouraged by the success of The Best Exotic Marigold Hotel, and by the rumours of a sequel. Even the recent return to TV of Birds Of A Feather is good news, she thinks, its female stars now being in their 50s and 60s. What about Downton Abbey, I ask. Scott Thomas's great heroine, Maggie Smith, is flourishing in the drama at 79, and so is Shirley MacLaine. Scott Thomas starred in Downton creator Julian Fellowes' first feature film, Gosford Park – another country-house period drama – so I ask if she could see herself joining the cast. She shakes her head.  "I can't do miniseries. Once you've got the characters, once you know who they are, they're going to repeat themselves, aren't they, for the next five years? It just goes on and on and on. I get terribly bored. Series bore me."  She is also rather weary of being typecast, playing quiet despair and heartbreak with the particular strain of dignity first showcased in Four Weddings. Scott Thomas hasn't had to audition for a part in years, but as a consequence gets "asked to do the same things over and over, because people know you can do that, so they want you to do that. But I just don't want to pretend to be unhappy any more – and it is mostly unhappy."  I suspect she is cast in unhappy roles because she has a gift for looking so compellingly beautiful when miserable. "Thank you very much, I'll try and make the most of that," she jokes. She has another theory: "Although I don't really think of myself as a sad person, there has been a lot of sadness. I've known a lot of sadness; I know what it's like to be sad. So I guess I have access to that, and can tap into it."  Scott Thomas was born in Cornwall in 1960, the eldest of five, to a former drama student mother and Royal Navy pilot father who died in a flying accident when she was five. Her mother married another pilot, who died in an almost identical accident when Scott Thomas was just 11. She was sent to a girls' boarding school, before enrolling at drama school in London, where she was told she would never make the grade and so decamped to Paris to be an au pair. There she studied drama again and was spotted by, of all people, Prince, who promptly cast her as his love interest in his vanity project Under The Cherry Moon. She was, critics agreed, the only decent thing in it, and has been working more or less nonstop ever since.  But, by her own account, she was not a happy young woman, and spent several years in analysis. She has been free of depression for years now, and comes across as conspicuously at ease with herself: grounded, good-natured and unflappable. Her comments about all the things she no longer likes about her film career may make her come across in print as rather grumpy, but her critique seems to derive from a newfound sense of liberation and self-belief. She is no longer the guarded, rather remote character who – as she herself says – invariably used to attract freezing water-based metaphors. In recent years, interviewers have remarked on how much she has thawed, and it would be easy to assume this has something to do with her 2006 divorce. In fact, she says, it was her rediscovery of theatre just over a decade ago that transformed how she felt about herself and her work.  In 2001, she played the title role in Bérénice on stage in France, having not been on stage for more than 15 years. It was a revelation. "I suddenly felt independent. You could walk on stage and you could stand on your head if you really wanted to. No one's going to say stop, don't do that, that's a ridiculous idea. There's this feeling of independence and trust – I could give myself permission to play things in a certain way and see if they worked or they didn't. I could trust myself."  The trouble with acting in films, she goes on, is that "you're constantly being told what to do. 'Move your head that way. Can you cry a bit more? Can you do this, can you do that? Oh, that was lovely, that was amazing, that was beautiful.' And sometimes you think, that wasn't amazing and wonderful and beautiful, it was just a look. But you've got this person saying it was. And then it's taken away from you, and it's all mixed up and made into something else. Basically, when you are acting in a film, you're giving the director the raw material to make the film. But when you're acting on stage, that's it. And that's when you discover that you can really do it. It's this word 'trust' that keeps coming to me. It's not a question of whether one person is conning you into thinking you can do it, saying, 'Oh, it was beautiful.' On stage, if it works, it works."  If Scott Thomas used to come across as somewhat aloof, she thinks this had a lot to do with her own ambivalence about her acting, and self-disdain for the narcissism of a career spent chiefly sitting around in trailers, waiting to be told what to do by a director. Since Bérénice she has starred in five West End plays, including two Chekhovs and two Pinters, won an Olivier award for best actress, and every time she talks about being on stage, her face lights up with excitement.  She says she will still make some films, but only those she absolutely can't resist. For the first time since her 20s, she is getting accustomed to the novelty of choice, for her daughter and eldest son are now in their 20s themselves, and her youngest son is 13, so the financial pressure of supporting her family is beginning to ease. She met her former husband, a gynaecologist, when she was at drama school, but since their marriage ended she has been facing a new freedom she's still not entirely sure what to do with. "I'm starting to think, hang on a minute, soon my youngest is going to go off and do whatever he's going to do. But this is what's quite fun about being me at the moment – making one's own choices, for reasons of one's own, and not trying to please a career person, or make money. I guess that's a sign of maturity, I think."  She and her ex-husband share joint custody of their youngest son, and although that makes life "much more complicated – you have to make appointments to talk to the father of your child, which is a bore", relations are amicable. "I know for so many people it hasn't turned out right, and they can't get the other partner to engage, and that sounds like hell. But I've been very, very lucky. Married the right man, had babies with the right man." She grins cheerfully. She does feel lonely sometimes: "But I think everybody does sometimes. I used to feel lonely when I was married and had three children, you know." The worst times are August, when Paris decamps en masse for the summer holiday. "Everyone's gone, you're on your own, and it's agony. You're trying to book a hotel, but you can't because the hotels are all full of these fucking families!" She laughs.  But the biggest challenge is deciding where she should even be. She has often said she feels more French than English, so I'm surprised to hear her say she is seriously thinking of returning to the UK in the not too distant future. Her adopted country appears to be, she says sadly, in terminal decline. "I think so. Everyone says it is, and it's quite believable. Five years ago, I would have said great schools, great education, great transport, trains are wonderful, roads are great, all that kind of thing – great, great, great. You can't really say that any more. And at the moment I'm really worried about the rise of antisemitism in France, which is just really unbelievable." Someone, she says, has got to stop National Front leader Marine le Pen.  Then again, she confesses, she doesn't actually vote, because she has never got round to getting a French passport. "It's about split personalities – it's about my life already being complicated enough. I'm splitting into a million pieces with this job I do, and I feel like I live in two countries, so having two nationalities as well feels too much. But I feel flaky not voting, I'm sort of ashamed of it."  What does she make of the revelations about François Hollande's private life? "Oh, I would really prefer not to know. In France, we are all shocked that the press has been so invasive, because there is a very strong – and I think quite right – belief that a private life should be private. But where I come a cropper is with this 'first girlfriend' business. I'm rather old-fashioned. This idea of being first girlfriend," she says, beginning to laugh, "is just bollocks to me."  • The Invisible Woman opens in London next Friday and nationwide on 21 February.
She's the go-to actor when it comes to elegant despair – but after 20 years, Kristin Scott Thomas is done with making movies. She talks to Decca Aitkenhead about shambolic film sets, French mistresses and the joys of saying non