Clémence Poésy made her name as an exchange student in the Harry Potter films. But she's also been Mary Queen of Scots, a hitman's girlfriend and a Gossip Girl. Now the quintessentially French actor has nabbed a part in the controversial new Danny Boyle film. But she hasn't always been this popular…

If Clémence Poésy was not French, they would have to invent her. She is the embodiment of all that is quintessentially Gallic: slouchily beautiful with an effortless pout and emanating a dishevelled glamour that matches her tousled hair. When she walks into the studio where we are meeting, she is wearing an oversized man's jumper and biker boots with the kind of nonchalant chic that makes most Frenchwoman's clothes seem an exercise in unwitting sexiness. She slides into her seat like a Burmese cat seeking out the most comfortable spot on a cushion. Then, when we start to talk, she admits that her favourite writer is Rimbaud. Well of course. Who else? "That goes back to being a teenager," she explains in gently accented English, sipping on her coffee. Inevitably, she takes it black.

In spite of the inescapable Frenchness, the bilingual Poésy is quietly edging towards stardom after a series of English-speaking roles in some major box-office hits. Perhaps best known in the UK for her role as Fleur Delacour, the glamorous French exchange student in the Harry Potter films, the 28-year-old also played the enigmatic love interest in the critically acclaimed 2008 film In Bruges, opposite Colin Farrell. Martin McDonagh, who wrote and directed that movie, said Poésy "could access a playfulness, a sweetness that belied her stunning looks" and it is true that her prettiness manages to be both obvious yet simultaneously difficult to pin down: a chameleon-like quality that means her roles thus far have been varied – from playing Mary Queen of Scots in the 2004 BBC mini-series Gunpowder, Treason and Plot to a regular part in the glossy US teen drama Gossip Girl.

Clemence Poesy with James Franco in Danny Boyle's 127 Hours. Photograph: Capital Pictures

In January she will appear in Danny Boyle's new film, 127 Hours, which tells the true story of climber Aron Ralston (played by James Franco), who became trapped by a boulder in a canyon in Utah while hiking and was forced to cut off his own arm with a penknife to escape. Poésy plays Rana, the ex-girlfriend who appears in Ralston's dreams and offers some respite from the plot's unrelenting grimness through a series of lingering close-ups of her ethereal blonde hair and mascara-smudged eyes.

Is she aware of her own beauty? "Not really. For me, it's about not being too aware of what you look like because if you are, you're trying too hard and I don't think that actually makes you look good… I've known from very early on that I don't look perfect. In family photos, I always looked scared, like I'd lost something whereas my younger sister [Maëlle, who is also an actress] was always making adorable faces."

She says that 127 Hours, Boyle's highly anticipated follow-up to the Academy Award-winning Slumdog Millionaire, was a fairly frenetic filming experience. "Danny is very intense. He sets a rhythm and it felt like everyone just had this continuous energy… He kept on putting things in to make it more difficult for himself. So he recreated this canyon in the studio and he did it so accurately that there was actually nowhere to put the camera that was comfortable." She laughs. "I think he puts his energy into every film he does and he takes you with him."

Although a handful of audience members have fainted during early screenings of 127 Hours – the scene where Ralston grits his teeth and snaps through his own bone is an extraordinarily visceral cinematic experience – Poésy insists she did not feel squeamish when she first saw it. "Really, it's a film about how far you can go for life, for love," she says, with an insouciant little shrug.

"It's not as if it was a big part of me": Clemence Poesy with the cast of Harry Potter. Photograph: Kirsty Wigglesworth/AP/PA Photos

Presumably, after learning to scuba dive for Harry Potter and the Goblet of Fire (in which her character endured an extended underwater swimming race in order to retrieve a golden dragon egg), she is used to suffering for her art. Does she feel bereft now that the Harry Potter franchise has come to an end? "Not really. I was just in one film and then I came back for the last one. To me, it was simply one of the experiences I've had. That was it. It was fun, but it's not as if it is a big part of me." Still, she keeps in touch with Emma Watson and sees her occasionally on the front row at fashion shows – Poésy is something of a muse to designers including Karl Lagerfeld and Balenciaga's Nicolas Ghesquière. "Emma's a sweet young woman and very bright," says Poésy, as though talking fondly about a particularly sprightly grandchild. "I like Emma's new haircut. It's very Jean Seberg."

For someone who seems so at ease with herself in front of the camera – Poésy also models regularly for magazines including Vogue and i-D – it is surprising to learn that Poésy felt herself to be miserably unpopular at school. Born in a southwestern suburb of Paris to a theatre director father and a mother who is a French teacher, Poésy attended an alternative school in nearby Meudon, where she had "a hard time. I just wanted something else. The relationship with the other girls my age was not so nice."

At home, her artistic parents refused to allow their daughters to watch television, insisting the TV set was broken – a claim that was somewhat undermined when Poésy walked in one day to find her father watching a tennis final. "And I know it worked because I had it for two years after I moved out of home," Poésy says. Then what happened? "I threw it away," she says loftily. "I listen to the radio now, first thing when I wake up."

Her salvation as a teenager came when she went to Canada at the age of 13 on a month-long school exchange. She found herself the centre of attention and discovered that popularity was an almost entirely random accolade. "I met wonderful people. We started art groups together because I loved to draw and I realised, 'You are worth something that those girls [back in France] don't see in you.'" On her return to Paris, she moved to a bilingual school and by the age of 16 she had enough self-confidence to call up an agent and, with the support of her parents, launch her acting career.

Although she has enjoyed a fairly successful few years since then, Poésy is not entirely convinced she wants to act forever. "There are very few actresses who can grow old and still get exciting parts… I think it's very important for me to have wider horizons, rather than just waiting for calls. I'll see what happens. I've been doing it for 11 years and there are a few things I'm getting slightly better at. The more you live, the better an actor you are, but maybe I'd like to do something else on the side. Something to pay the bills as well."

It is a profession, she says, in which there is a pervasive pressure to conform. "You're constantly reminded to look a certain way. I've spoken to friends who tell me they're freaking out about a job interview and I realise that I spend my life having a job interview [through the audition process]. There is no moment where you can rest and think: 'Wonderful, I have that job now. I'm going to spend five years here.' There's a constant judgment on your work that's very strongly related to what you are."

It is, perhaps, ironic that the young girl who was desperate to be liked at school has now found herself in a career where success is so connected to the outward manifestations of popularity. "Yes, it means you need to be a bit strong and surrounded by people who love you for who you are," Poésy says, gazing into the mid-distance. "Because when you film someone you steal a bit of what they are."

Blimey. She is terribly serious. But then, just as she is about to go, the reason for her mild preoccupation becomes apparent – it transpires her handbag was stolen yesterday while she was in Pret a Manger and she has lost her wallet and her phone. The thief has since then been leaving comments on her Facebook profile saying how sorry he is. "It's weird, huh?" she says, a touch nervously. "I have a fake name that I use and he's found it and is now writing all these messages." Yes, I say, that does sound odd and not a little stalkerish. What kind of bag was it? "It was my first Chanel," she says, with a sad little smile. Still, given that she is Karl Lagerfeld's muse, I am sure they will send her a new one. "Yes," she says, smoothing down her skirt, tying up her hair loosely in a chignon and getting ready to leave. "Chanel have been very good to me." She walks out of the room with a smile, still effortlessly beautiful, and is whisked away by a waiting taxi.

If they could see her now, I imagine those girls at school would be seething with jealousy.

127 Hours is released on 7 January. Elizabeth Day is a writer on the Observer. Her novel, Scissors, Paper, Stone, is published by Bloomsbury

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