Hollywood's love affair with Marion Cotillard

If you could enter anyone's head and watch their dreams – as Leonardo DiCaprio can in the film

We have learnt to expect the unexpected from Marion Cotillard, France's brightest star. At 34, as well as being an actress in full bloom, she is a committed Greenpeace member, and has just returned from a week investigating deforestation in the Congo. She is also, thanks to what she calls her 'intuitive relationship with fashion' the current 'Lady Dior' (ie, the face of Dior). Nicole Kidman, who became a friend when they were working on Rob Marshall's film Nine – on the set of which Marion ruthlessly oversaw an overhaul of the recycling facilities – told her earlier this year that she was 'a paradox'. 'You have this fairy quality,' she said, 'like you're flitting through trees and stars, and at the same time, you're really grounded.'

Marion is certainly a shape-shifter. She became, unforgettably, the shrunken and ageing Edith Piaf for the Oscar-winning La Vie En Rose, curving her spine to lose a foot in height, and was equally convincing as a pneumatic gangster's moll in Michael Mann's Public Enemies, with an authentic Louisiana Bayou-Cajun accent that she honed obsessively. 'I wanted to give everything I could; I mean, I could've given my blood.'

On the red carpet she has style, favouring old-fashioned gowns in delicate, muted colours. 'I have always been fascinated by the styles of the 1920s, 30s and 40s. I don't know where it comes from but I have a sentiment towards those decades – for the fashion, the design, the architecture.' She wears discreetly deployed Chopard jewels and, on her arm, a supportive, bearded boyfriend, the actor-director Guillaume Canet – with whom, these past three years, she has quietly formed a power couple to overshadow Monica Bellucci and Vincent Cassel. And yet, when it comes to being photographed without a character to assume, she becomes shy. 'I always feel very uneasy when there is no story to tell, when a photographer says: "Be beautiful and pose".'

As is traditional for French actresses, Cotillard combines glamour with thoughtfulness. Aged nine, she underwent something of an existential crisis. 'I was shy. I was more than shy. I didn't know where was my place anywhere,' she has explained, in her idiosyncratic English. 'In school, with friends, with the other children... I started thinking about why I was here, and I couldn't find any answers, so it was very disturbing for me. [It was] as if my brain had lost its innocence too quickly.'

She grew up in a happy, creative home in Orléans, south of Paris, with theatre actor parents (her father studied mime under Marcel Marceau's teacher Etienne Decroux) and two younger brothers identical twins Guillaume, now a screenwriter/director living in Paris, and Quentin, a San Francisco-based artist. Her parents, Jean-Claude Cotillard and Monique Theillaud (who now goes by the name of Niseema after a spiritual rebaptism) moved from Paris to a suburban multicultural tower block, and finally came to rest outside Orléans in a village called La Beauce, in 'a beautiful house with a huge garden'. Here, Marion felt 'connected to nature and the seasons' – the beginnings of her environmentalism. Staying in Brittany with her much-loved maternal grandmother (now aged 101), she admired the way that when she cooked, she didn't waste anything. 'My parents always raised me to believe that the most important thing was respect. Respect the place you live, be aware of the impact you have on things.'

Her teenage years were troubled. 'I didn't like myself I hated myself. I just had to live and find something, so that I would do something good and finally say, yeah, maybe I'm not that uninteresting person I think I am.' That something was acting. At 15, already attuned to drama from performing with her parents' touring company, she became the youngest ever student to attend the Orléans Conservatoire Dramatique, where her father taught. 'There, I found my place.' As she has said in the past: 'As a teenager I didn't want to be me; I wanted to be many different people. Maybe I realised that they all lived inside me and that if I managed to connect with them, they would become aspects of me.'

Her first roles in her early teens were in TV dramas such as Highlander, a spin-off of the Christopher Lambert films, and the zany Luc Besson film comedies Taxi, Taxi 2 and Taxi 3. They were hugely popular but it was not the work she dreamed of. 'I didn't want to get bitter waiting for something to happen, so when I was 27 I told my agent I was going to work for Greenpeace. He said, "Just have this one meeting." It was with Tim Burton for Big Fish and I got the part.'

Her role as Albert Finney's confidante in the surreal fairytale brought her international attention. The director Olivier Dahan saw something of Edith Piaf about her eyes, and co-wrote the script of La Vie En Rose with her in mind. 'When I was working on La Vie En Rose, I connected with an energy that was not entirely mine,' she once told a reporter. The process of becoming the French torch singer was highly technical, spanning months of research, lip-synching and experimenting with make-up, shaving her hairline and eyebrows and using subtle prostheses to remodel her face. But there was also a quasi-mystical surrender. 'I learned how to make the space inside me for her to come in,' she has said.

After the film wrapped, she still found Piaf's phlegmy cackle in her voice. She went, with Canet, to Peru and to the Amazon; then to Bora Bora with her best friend Geraldine. She realised, ultimately, that she hadn't been able to let go of Piaf because 'Piaf had been abandoned as a child; her greatest fear was to be alone. Now I didn't want to abandon her. I was finally able to say, she's been dead for 40 years – it's OK.'

For La Vie En Rose, Cotillard won all the prizes going: an Oscar, a Bafta, a Golden Globe, a César, and 17 more. It was only the second non-English speaking performance to win a Best Actress Oscar (the first went to Sophia Loren in 1962 for Two Women). Cotillard keeps the trophies in a corner of her Paris garret alongside the Trophée Chopard, awarded to her in 2004 for the film Jeux d'enfants in which she starred with Canet. 'Receiving the Trophée Chopard at the mythical Cannes festival was a very significant moment for a young actress,' she says. She wears Chopard to this day, and particularly loves their 150th anniversary Animal World collection, developed in association with the WWF. 'Anything we can do in favour of the defence of life and the environment is always precious.'