The Joseph Conrad short story on which Patrice Chéreau has based this superbly orchestrated and pleasingly filmic portrait of the disintegration of a marriage in pre-World War I Parisian high society is 'The Return'. It's a tale which itself offers hints of Tolstoy's near-contemporary novella, 'The Death of Ivan Ilyich' (on which Bernard Rose based his excellent 2000 film Ivansxtc). Both Conrad and Tolstoy, and now Chéreau, present portraits of powerful men and women at the cusp of modernity who enjoy privilege but whose wealth, pride and ambitions - inherited and sought - veil them from a sham at the core of their lives.

Pascal Greggory is Jean Hervey, a stern and taciturn man with sterner whiskers, whom we meet emerging from the rush-hour crowd one Wednesday afternoon as he returns to his grand city home and his wife of ten years, the beautiful, controlled Gabrielle (Isabelle Huppert). The voiceover tells us that he's a 'man with money and friends', 'tall and healthy', and a flashback (from black-and-white to colour, a jolting mechanism used throughout) to a week earlier shows us Jean and Gabrielle as they host one of their 'famous' weekly dinners at which a gaggle of the city's beau monde share witty exchanges, play cards and gossip tastefully. Back to the present, and on this evening, Jean's house is empty and lifeless, give or take the odd servant, and Chéreau and his cinematographer Eric Gautier lend the house a cool, shadowy light, all dark blues and chilly greens. Soon, Jean discovers a letter from his wife, and Chéreau shocks us - and Jean - out of our complacency with a nifty piece of sudden camerawork involving mirrors. The shock causes Jean to drop his whisky glass and the film jumps from black-and-white into colour. Gabrielle has left Jean. Then, minutes later, the news still wet on the page, she returns home. She's changed her mind. And she's transformed her world of manners and rituals and habit and forced her and her husband into a post-mortem of a loveless, sexless marriage.

The rest of the film deals with the fallout from Gabrielle's decision first to leave and then to stay. Awkward discussions are punctuated with attempts to live life as normal: a dinner party, the rituals of eating and dressing. Most satisfying is how Chéreau decides to give equal measure to dialogue, photography and music. A chilling orchestral score slips in and out, and some classic, sweeping camerawork uses the labyrinthine house and its chorus of servants to stress the social fallout of Gabrielle's decisions. Huppert and Greggory, with credible and precise dialogue, handle excellently the contrast between the formal and the intimate - an opposition which is powerful and consistently uncomfortable. One minute Jean is considering out loud and with cold reason how to hide this disgrace from his friends; the next Gabrielle is raking over her sexless marriage and declaring that 'the thought of your sperm inside me is unbearable'.
Chéreau and regular screenwriter Anne-Louise Trivdic tweak Conrad's original a little - moving the action from London to Paris, shifting the story forward a few years to 1912 and giving more of a voice to Gabrielle - but this is a careful and cinematic adaptation that rings with painful truth.

Even if she is nothing like the fearsomely unhinged characters she regularly plays, Isabelle Huppert is quite a scary prospect. As an actor, she is dauntingly accomplished - rarely does a Cannes film festival go by without her: she has won the best actress award there twice (for The Piano Teacher in 2001, and in 1978 for Violette Nozière), and is a former jury member. She is also fiendishly clever. When she guest-edited the French film bible Cahiers du Cinéma in 1994, she chose to interview notably tough subjects - the philosopher Jean Baudrillard and the postmodern writer Nathalie Sarraute. She has a reputation for being extremely private: despite her sparrow-like build, she has been known to devour lazy journalists whole.
We meet in a London hotel, and she is dressed casually in jeans and jacket, a black scarf round her neck. She has just had lunch with the French ambassador. Tonight she is giving a talk at the National Film Theatre, and she's got a hairdresser's appointment, so, she says, we don't have much time. She speaks excellent English. I find myself telling her she's quite scary.

"Oh!" she says, half surprised, half sympathetic. "After about five minutes you'll be reassured, I promise you." And she is, it turns out, a good listener, keen to play her part.

Huppert got her big break in this country in 1977 with The Lacemaker, in which she played a reserved hairdresser; in her 20s, Huppert was often cast as a shy, passive, but not entirely powerless beauty. Now, in her 50s, she seems to be specialising in lean, lonely women capable of gross transgressions. The director Claude Chabrol has used her to this end many times, from Violette Nozière, in which she played a teen murderess, to 2000's Merci Pour le Chocolat, where her wealthy heiress is still dishing out the poison. Elsewhere she has gone further, most memorably in Michael Haneke's The Piano Teacher, as a repressed spinster doing unspeakable things with razor blades and broken glass. She topped even that in 2004's Ma Mère, playing a bisexual bohemian in an unhealthily close relationship with her son. She has appeared in lighter roles, in Hal Hartley's Amateur, David O Russell's I Love Huckabees, and François Ozon's 8 Women - but even then she is playing comic variations of these outsiders. If she was anything like them, she wouldn't be allowed to walk the streets.

So how did Huppert become the queen of arthouse pyschosis? "There is something objective about it and something subjective," she says. "The objective part of it is that these roles are being given to me, so I take whatever comes. And the subjective part would be that, even if they were simpler or less scary, I think of the little story about the scorpion who stings the crocodile when he helps him cross the river. The crocodile says, 'Why did you sting me?' and the scorpion says, 'It's in my nature.' I guess ... that's my nature. Not that I sting, but I like to bring out some kind of shading. Some kind of contrast."

Of course, half the trick of these roles is being able to play normal. Huppert can act depraved, heartless, voracious, repressed - but she also makes these characters believable, even sympathetic, human beings.
"I never feel I am playing characters," she says. "I play certain states, certain ranges of emotions, certain feelings. The contours of a character are something very vague. Initially, when I read a script, I have certain pieces or images that come up to the surface. I guess the difference would be between abstract painting and figurative painting. An established character would be more like a figurative painting: you have to paint this room, you have to paint the curtains, and so on. I think it's more interesting to consider acting as non-figurative painting. As a canvas on to which you throw things. It can be a colour, it can be a rhythm, it can be music. And it's also a vision of yourself."

She gives an example. "When I read the script of Ma Mère, I thought it was very difficult to do. Because in principle it's a story about incest, even though it's much more than this. I hesitated a lot over whether I was going to do it, and I had this very fugitive impression of the role - for me it was like a flame. I saw this woman as a little bit of fire, being very unstable, rickety, not very steady on her feet. I had this vision of my body - sometimes that's enough."

Huppert says she would never want to direct a film, but that, in effect, she directs herself. Chabrol once said of her: "All I need to do is say where I am going to put the camera for her to guess how the scene is going to be shot and what will be her place in the frame. With her, I never have to explain anything." Other directors do the same. In most of Huppert's films the camera comes to her, rather than the other way round. There is a key scene in The Piano Teacher where the camera simply watches her face while her future amour fou, Benoît Magimel, auditions at the piano. For several minutes she just sits listening, but as the Schubert rises and falls in the background, she hints with minute facial movements at an equivalent range of feelings roiling under her impassive facade. It is moments like these that challenge our assumptions about acting, and being acted to. Is her every action deliberately calibrated to register the desired effect - or is she simply sitting there listening to the music, allowing us to project a story on to her almost blank features? She tells me that scene took only a few minutes to shoot. She had a plane to catch.
In her new film, Gabrielle, directed by Patrice Chereau, there is more close-up work, and an even more challenging intensity. Based on Conrad's short story The Return, it is a concerted re-imagining of a period-chamber piece, using modern techniques (switches from colour to black-and-white, temporal glitches, sudden bursts of text on to the screen) to heighten an existential domestic showdown, sparked by Huppert's decision to return home a few hours after leaving her husband. No prizes for guessing who gains the upper hand. The film culminates in a hideously chilly and unerotic sex scene, of the type, yet again, one could only imagine Huppert playing. "He takes her weakness and she takes his strength," she says. "She becomes the man, with whatever that implies - the cynicism, the capacity to lie. When she asks him to sleep with her for the last time, he says, 'Can you do it without love?' She makes a real human being out of him by forcing him to taste the poison of truth." By Huppert's standards, this counts as a happy ending.
Though she refuses to talk about her personal life, it is by all accounts a very ordinary one. She is happily married, to director Ronald Chammah, and has three children. Her eldest daughter, 23-year-old Lolita, is also an actor, and appeared alongside her in 8 Women. But the more you try to get Huppert to talk about anything other than acting, the more guarded she becomes. She doesn't like celebrity, and she doesn't like being photographed - although she did once publish a book of portraits of herself by famous photographers (Annie Leibovitz, Helmut Newton, Henri Cartier-Bresson), Isabelle Huppert: A Woman of Many Faces.

Of course an aura of mystery is almost compulsory for a grande-dame, but with Huppert you sense there is more to it than that. She gives herself away so much on screen, and takes such risks, that perhaps she wants to keep something back when she's off it.

"I have attempted to keep this constant link between the films and roles I choose to do and my own persona," she says. "That's why it's so difficult to choose what film to do, and the people I work with. It's like there is no division between doing a film and your own life. Making movies has so much to do with privacy, with intimacy," she says. "I think that in order to make a film I have to feel this potential, this possibility, of being private - more private and intimate than at home. It's not a public matter for me, being an actress - it's a really private matter." She is aware of the paradox, but it's one she likes working with. "Most of the time that's what cinema is. It's public, it's private, and that's why movies create such an emotional impact. It's also very pragmatic. It's just work we're talking about. The rest belongs to me."
Already Huppert has made more than 80 films, and says she has no intention of stopping: when she is not shooting, she is on stage. She completed another film with Chabrol last year, their seventh, and has other projects in the pipeline. Right now she's acting on the Paris stage in Quartet, an adaptation of Les Liaisons Dangereuses. "I like to work, that's for sure. And I also know what it means not to work," she says. "Sometimes it would be nice just to see life differently, rather than always through the prism of the screen. But for the moment that's the way it is".