Marion Cotillard: 'I'm really normal! Well, not that normal. I'm an actress'

owards the end of an hour in her company, during which she has skirted round the topic of what she is doing next ("I have a project for the summer. Different thing that I don't wanna talk about..."), and in which she has sniffed, snorted and spluttered her way through her flu, I ask Marion Cotillard what I suggest is a stupid question. In her new film, the ensemble drama Little White Lies, made by her boyfriend, the actor-turned-director Guillaume Canet (Tell No One), she plays a Parisian ethnologist. But her previous few roles have all been English-speaking parts. Is it more difficult for the Frenchwoman to act in English than in her native language?

"Oh, it's not a stupid question," she replies eagerly. "Oh no, no, no, it's far from being stupid. Yeah, it's totally different. And it's more work. But it's something that I love to do. I love to work on the detail of the sound, how you stress a word, the meaning of the rhythm of what you k say. It's very, very interesting. And I love the English language. But yeah, there's a difficulty added to..."

She pauses. The Oscar-winning star of the Edith Piaf biopic La Vie en Rose (2007), last seen on screen in Christopher Nolan's mind-melting and box-office-wowing Inception and, before that, as Johnny Depp's girlfriend in Public Enemies, immerses herself in her roles. Completely. During her time as Piaf, she sang every day for seven months, beginning each morning of the three-month rehearsals and four-month shoot with a tilt at some of the Little Sparrow's canon. While making any film, she adds, she doesn't even read books. She can't be distracted from the job at hand. "I'm not the kind of person who can do a lot of things at the same time."

Thus, during the making of Public Enemies – Michael Mann's film about the American gangster John Dillinger (Depp) – she kept her entire life English: off set, she thought in English, talked in English. "It was really, really weird," the 35-year-old says in her rhythmic, repetitive, emphasis-heavy speaking style, "because I really wanted to have this perfect accent and I knew that it was not possible to have a 100 per cent perfect Chicago accent from the 1940s. It was really, really hard. So I stopped speaking in French for four months, and I started to speak in English with my friends [from home]. Which was very weird."

Such devotion can come at a cost. All that singing she did for La Vie en Rose? Cotillard's voice wasn't even used in the film's musical moments. She just did it to better get into character, "to learn the technique" of lip-syncing, "which is beyond difficult". The director used Piaf's original recordings. Or, for the scenes where the impoverished, unknown Piaf sings for her supper (or, for another bottle of hooch – she liked a drink), an impersonator.

"Ah, no," the actress says to me with a shake of her head, doing the singing herself was never really an option. "No. No, no, no, no, no. Because I had only three months when we finally got the funding, to prepare for the role. So it was... freaking short! So I was frustrated, I have to say. Not that I really wanted to sing, but I wanted at least to have the time to try. And I didn't."

After completing work on the film, and given its resultant success, did it take a while to shake off Piaf? "Ooooh," Cotillard breathes, "a while was long. A while was nine months. It was terrible. And I'm ashamed to say it, but that's what happened."

She was, she admits, haunted by the star, one of France's greatest cultural icons. "I had this weird feeling. And I'm very sane! I don't live with my role, I'm a really normal person. Well, I'm an actress, I'm not that normal! But I tend to be really normal." She smiles. "But I couldn't let her go because her biggest fear when she was alive – of course, we don't have fear when we're dead – was to be alone.

"And then when we finished the movie, I didn't want Edith to be alone. That's crazy! Then, suddenly, going back to my life and leaving her alone... It just took time for me to realise that she had been dead for a while." Now Cotillard laughs. "So it was OK! She was not alone! She was gone."

The ghost was exorcised, and Cotillard's body hair returned – to play Piaf, she'd had her hairline heightened and her eyebrows removed. ("Shaving doesn't make them thicker, it just makes it ugly when it grows back.")

And physically, she was, finally, no longer bent out of shape. Trying to appear like the gawky, thin Piaf across the span of her adult life (she died in 1963, aged 47) meant Cotillard had tried to "shrink" her real height by nine inches to somehow resemble the 4ft 8in chanteuse, putting a great physical strain on the actress.

"But that's something that I really like, and I think it's easier for me to have a character that is really far from me. And, for example," Cotillard adds in her chewy, American-accented English, "with Little White Lies, the difficulty was that the character was not that far from me."

It is a bright but cold December Paris day and Cotillard is spending it conducting a couple of international interviews in a hotel. It is rather early for her to be doing press for a film not due to be released until the spring. A few days later, a possible explanation emerges: it is announced that Cotillard and Canet are expecting a baby. Perhaps the actress, chary of discussing her private life, didn't want to be asked about the pregnancy by journalists. If that was the "different" kind of "project" for the summer that she had been referring to, fair enough.

Little White Lies is about a band of thirty- and forty-something Parisian friends. Every summer they journey to the holiday home of Max, a restaurateur, on the Atlantic coast at Cap Ferret. The film opens with one of their number carousing in a nightclub. He exits in the early hours, high on cocaine and booze, jumps on to his scooter, zips off through the quiet Parisian streets, and then... Well, let's not spoil it.

Canet filmed this extended scene – he also wrote the script – in one remarkable shot. It's the only stylised bit of the film. The remainder recounts, in naturalistic, empathetic, keen-eared detail, the group's discussions of whether they should still depart for their annual group vacation – then, once they are on the coast, the director's non-intrusive cameras linger on their thoughts about their actions, the differing states of their relationships, their views on where they're going with their lives.

It is, then, a Gallic take on The Big Chill, a reference Canet is happy to acknowledge – and a notion underlined by the fact that most of the actors were friends before filming began. Cotillard said that her boyfriend – they've been friends for 15 years, and acted together in the 2003 film Love Me If You Dare – gave her a choice of roles to play. She opted for the part of Marie, the scientist who spends much of her time abroad studying humans, has no-strings-attached sexual relations with men, but is remote from close relationships.

"I chose Marie, so I thought, well, there must be something that I have to explore there."

Hiding from people in a crowd – is that something she could relate to? "Um... yeah," she says crisply. "Yeah, because I think an actor is kind of an anthropologist." Born in Paris to actor parents – her father had a company that staged plays for children – Cotillard thinks that actors "study human soul, human heart, human behaviour. We try to understand this in order to be able to be [true to life]. And when I was a teenager I was... kind of special. I was very scared of other people. So I would hide. But I was more, I was more, let's say, dark."

Melancholy?

"Yeah. I had a thousand per cent of my teenage angst!" She laughs, sniffs and sips at some water. For all the heaviness of her illness, she is remarkably perky. "I started to ask myself very early why I was here. And I think it makes your innocence go away faster, to have this obsession of getting an answer about what are you doing here."

Cotillard's innocence was also rudely interrupted by the international success in La Vie en Rose. Her Oscar win made her a red-carpet fixture (she became a model/muse for Dior) and, after a few years of success confined to France (kickstarted by 1998's Taxi), an international star.

But she didn't like "to be the one under the light", so took smaller parts in bigger movies such as Inception and Public Enemies. And she spent time singing and playing bass/keyboards with a below-the-radar group, Yodelice, comprised of Parisian musician friends.

The worst example of her new-found "status": shortly after her February 2008 Oscar win, she was pilloried for comments she had allegedly made on a late-night French chat show a year previously, in which she seemingly cast doubt on the veracity of the 9/11 attacks.

Cotillard sighs when I mention this. "You know, I know how the media work. And I have to be honest: it was really stupid for me to talk about something that serious in a TV show at one in the morning. But we were talking about something totally different and I gave an example of what I had seen. And it was not very smart. But still: what was written out of it was very different from what I said."

It was reported that she'd said the Twin Tower attacks were a fake... "I didn't say that," she interjects. "The first reason is that I know people who have lost members of families or friends that were in those planes. So how could I believe in the conspiracy theory? It's nonsense."

After Little White Lies, Cotillard next appears on screen in Woody Allen's Midnight in Paris and Steven Soderbergh's Contagion. The latter, which she shot in Hong Kong, also stars Matt Damon and Kate Winslet, "two of my favourite actors ever". There are ongoing rumours that Christopher Nolan wants to work with her again in the next Batman film, The Dark Knight Rises (could that, in fact, be the "summer project?"). But her part in Cosmopolis, a planned project with David Cronenberg, has hit the buffers due to problems with Colin Farrell's schedule.

So, with whom – and in what language – would she like to work next?

"I just wanna do movies," says Marion Cotillard with a shrug. "I don't have a plan. I'm so lucky to have the opportunity to work with some directors and some actors I wouldn't have dared to think I would work with one day. And now I know that everything is possible. So, it's very exciting. But I don't have in my mind whether I do an American movie or a French movie. It's just [that] the stories come and... if I recognised myself in the story, I wanna be part of it."

Another smile, another sniff.

"That's how it happens."

'Little White Lies' (15) is released on 15 April