We're sitting in The Pissarro Suite of the Hotel du Louvre, an old-style establishment full of wooden panelling and elegant patrons. The room has a view down Avenue de l'Opera that Pissarro painted when he stayed here, so I feel like I'm in epic company. Thank goodness Binoche is relaxed and casual. She's ensconced in an overstuffed lounge with a bottle of water and looks at least 10 years younger than her character in the film – also an actress. She listens intently and speaks with obvious passion. It's clear that this film is important to her.

"I dreamed about an idea of a film with three female characters," she says in fluent English, a hint of American rather than French in the accent. "But I didn't know what the story should be. When I thought about who I'd like to make the film with, it was Olivier Assayas who came to mind." Binoche and Assayas are old friends. They had worked together in 1985 when – aged 21 – played a young actress in Rendez-vous, a film that Assayas wrote. "I missed Olivier and gave him a call," says Binoche, "and he was very open to the idea. He said he'd try to write something. Two weeks later he called me and said he had the story." That's how things happen when you're La Binoche.

Although it would be another 18 months before a final script was ready, Binoche knew Assayas was on the right track. The story of The Clouds of Sils Maria– like Rendez-Vous – is about an actress, but this time a woman at the other end of her career. Binoche plays Maria Enders, an international star who decides to perform in a play she appeared in 20 years earlier. The play – to be rehearsed in a remote Swiss Village called Sils Maria – is about two women, 20 years in age difference, who are involved in a co-dependent and destructive relationship. Having previously played the younger role, which she identifies with, Maria must now play the older woman: the has-been, a character ultimately usurped by youth. In contrast to the more serious Maria, a flippant Hollywood starlet named Jo-Ann Ellis (Chloe Grace Moretz) is cast as the younger woman. Helping Maria through the personal challenges of dealing with her co-star, the role – and a messy divorce - is her assistant Valentine (Kristen Stewart), another strong, next-gen female. When Valentine runs lines with Maria to help her prepare for the play, it's clear that we're witnessing a reflection of their own relationship: Maria is dependent upon her tech-savvy and bright young American assistant for more than just the admin.

When Binoche finished reading through the final drafts of the script she "was blown away by the complexity of the film". She pauses then adds: "and by the task". "The task for the film and yes, hello, the task for ME!" She laughs boisterously, describing the eight-week shoot as "a whirlwind of having to be present".

Which leads us to talk about acting and preparation for a role. Many actors – particularly younger ones – expect to arrive on set with little or no rehearsal, relying on their intuition to get them through. Binoche had six weeks to prepare for her role – a process that she calls "work". "I'm not a magician. I have to make it real. So I work." This was very different to Stewart's approach. "Kristen doesn't really work like that," says Binoche. "She trusts her intuition and in particular the inspiration of the first take. Of course, we spent a lot of time together and talked about the relationship between the characters we were playing, and we rehearsed once. But I decided it wasn't her thing. Her choice is not to work very much in advance."

"Work" for Binoche is about finding the truth of what's written in the script. "I don't judge the writing," she says. "I have to find how it resonates with me. I have to find the real connection. After all, this character Maria doesn't have children, she's in the middle of a divorce – she's different from me. I've never had a career in America like her. But at the same time it was amazing that Olivier wrote this role as an actress – particularly because he wrote Rendez-Vous which is also about an actress. He explores ideas of time, maturity and how you cope with a switch of roles in time – I found that daring . But most of all I was touched by the idea of showing people a glimpse of what it costs to be an actor."

For Binoche, acting becomes an almost spiritual pursuit. "You serve something bigger than yourself." she suggests. "You are using your self, your ego, to move to another place – called art, called vision. It's a transition to another dimension and that's why it's so exciting. As an actor I have to be in the nakedness of my own being, and take the risk, find the new. That's why you have to know yourself. " Once again she laughs with gusto. "Ah, that's a timeless project," she says knowingly. "You can work until the end of your life on that one. You need knowledge of your body, your mind, and of anything that is sensation. It's as if you're opening the pores of your skin – everything has to be alive. And as you go through life, it's always changing. I really believe that it's only through difficulty that you really find yourself, because that's where you discover knowledge."

The Clouds of Sils Maria will, I'm sure, be a much studied film, such are its complexities and tangled themes. Binoche is an actress playing Maria Enders, an actress. Maria is performing a play she's done before, now from a new perspective, and has to adapt to the change that comes with age, with knowledge. Binoche recalls a moment when the characters are on the theatre set and Maria asks Jo-Ann, the young Hollywood star, to change the way she's acting because it doesn't feel right for Maria's character. Jo-Ann stares at her incredulously. " But we don't give a f--- about your character at this moment in the play," she replies with brutal and naive honesty. "That moment for Maria," says Binoche, "is like a death, but the kind of death that sets you free. Your belief system has to change as you go through life. You cannot go back to the past."

The Clouds of Sils Maria opens May 7.