Love-hate relationship

"Demandez le journaliste," one of her team tells her. The journalist doesn't mind in the least. Indeed, given the controversy that has followed Abdellatif Kechiche's near three-hour, 18-certificate exploration of a lesbian love affair, the request to light up is amusingly mild. What kind of way is that for a rebel to behave?

As it turns out, Exarchopoulos is sweetness and lumière, which comes as something of a surprise given the near nuclear-level row that broke out between Blue's stars and its director after the shoot had wrapped. We will talk about that outbreak of unpleasantness later, but for now, at 19, with her first major film behind her, Exarchopoulos is at that happy point where she is wild about the business and the business loves her right back.

I ask if she has had a chance to sleep between the States and the film's London Film Festival premiere. "A little bit, but with all the jet lag, in every country … But it's cool," she laughs, "we are here."

"Cool" is one of her favourite words. There is a translator in the room, but Exarchopoulos (her grandfather is Greek) is generally in too much of a hurry to wait. Her words pile on top of each other like puppies trying to feed from the same bowl. While this approach occasionally results in outbreaks of actorly gobbledegook, she has a way of cutting through the white noise now and then with a sharp, telling phrase. An impressive young woman is Adele E.

She appeared in her first film when she was 13 and the French film industry provided fairly steady employment thereafter. Then came word of a new drama from Kechiche, a French-Tunisian director with a reputation for being a bold chronicler of modern France. In the first auditions with casting directors, Exarchopoulos had to pretend to tell a friend that she had fallen in love with a girl. She had also read the graphic novel by Julie Maroh on which the film is based. Then it was time to meet Kechiche - again and again.

"He really wants to know you, because for him being an actress is about being instinctive. In France he is famous for taking people like me from the shadow to the light, people who have made nothing, and giving them the chance to lead a story," Exarchopoulos says.

What Kechiche remembers from those first meetings was cake. He has said: "I had taken her for lunch at a brasserie. She ordered lemon tart and, when I saw the way she ate it, I thought, 'It's her!'"

In the film, the hearty appetite of her character - also called Adele - is used as shorthand for her no-frills, working-class background, in contrast to the middle-class, artsy upbringing of her girlfriend Emma, played by Léa Seydoux, 28.

When Exarchopoulos discovered more about the film, that it was going to be an intense exploration of first love, she did not hesitate. "Abdellatif wanted to make people forget that it was two girls. For us it was just a love story. During the whole shoot we never spoke about homosexuality, lesbians, not for one moment. It's weird because when we won the Palme d'Or at Cannes it was the day when gay marriage was legalised in France, so everyone was talking about it, that this was a movie of its time. But in fact it was a beautiful coincidence."

Yet it is the subject matter -more precisely, the extended sex scenes between Exarchopoulos and Seydoux - that have caused such a stir. One lasts 10 minutes, and nothing is left to the imagination. There were no body doubles. "We were protected," says Exarchopoulos.

I'm puzzled. You mean "down there", I say, pointing. I'm conscious of speaking in a ridiculously exaggerated style, like Les Dawson's Cissie and Ada. She nods, totally unfazed. "Second skin." The things that can be done with plastic these days.

Does it annoy her that so much attention has been given to the sex scenes? "Yes, because it's reducing the movie, you know?" Getting the scenes to look so naturalistic on screen took a lot of time and effort. "At some moments we had to find a second breath. Everyone was tired. Abdellatif creates all the time. We had many takes because he wants you to abandon yourself, to not control your acting. He doesn't want you to be conscious about the movie, about him, about the situation, he just wants you to be in the moment. We really had to trust each other."

It is this intense style of working - one brief scene involved more than 100 takes, Seydoux has said - that seems to have led to the spiky moments between Kechiche and his actors. In an interview with the Daily Beast website, Exarchopoulos said: "[Kechiche] is a genius, but he's tortured." Seydoux said: "Thank God we won the Palme d'Or, because it was so horrible." Kechiche has rejected the criticisms.

But now tempers appear to have cooled. Did she fall out with him? "It's more complicated than that. In every work there is conflict - hopefully. It's the way it goes, you have conflicts with people you love. If you didn't care, you wouldn't bother to have conflict. Abdellatif asks a lot but he gives you a lot." Are they friends now? "Of course. For me he's like a spiritual father." Seydoux she regards as a big sister. The two have a new gig together, as the 2014 faces of the fashion label Miu Miu.

Exarchopoulos, who lives in Paris, has two younger brothers. Her mother is a nurse and her father a guitarist. Her mother was working when the film had its Cannes premiere, so dad went along. She had only seen the finished movie a day before, and said "everyone" was telling her not to let her father attend the screening as it would be too shocking.

"I said it's my first time in Cannes, it's my first lead role, it's my first famous movie, and I want to share my happiness with the people I love." Dad, in the event, was very proud of her. "My family are the only ones who don't talk to me about the sex scenes because they respect it and they know it's not true."

The post-Cannes frenzy has been one of plane rides and press, with so many journalists, she says, telling her that her life is going to change. She doesn't feel that way. "When I come home I'm no-one. I'm just a daughter." Yet she knows, also, that big things are expected of her now.

"You have the feeling that everyone is waiting for your next movie to say you were better in this one, you can't make everything. You're like yeah, but I'm 19, I have so many things to learn."

Blue Is The Warmest Color opens in cinemas on Friday