If you value your life, don't confuse the real Marion Cotillard with the characters she plays onscreen.
"When people ask me how I am like Edith Piaf, how I am like Luisa Contini, then I become very mad," she declared, eyes blazing. "I am an actress and they are characters I am playing. Why is that so hard for some people to understand?"
After five minutes in her presence, you not only understand her point perfectly, but you wonder how anyone could make the mistake.
Cotillard took the world by storm as Piaf in La Vie en rose, winning anOscar for her performance as the decrepit, drug-addled "little sparrow" who scorched the concert stage with her self-destructive songs.
And now, she is exquisite in Nine as the liquid-eyed Luisa, suffering through the infidelities and humiliations that her husband, internationally acclaimed film director Guido Contini, keeps publicly inflicting on her, until she can take no more and strikes back in a fantasy striptease number of unparalleled masochism.
But she's neither of these women. Nor is she the mischievous Fanny from A Good Year, the battered Billie Frechette from Public Enemies or ... well, you get the picture.
"What I love about my job and why I'm doing this is the desire to understand someone else. And you don't have to be someone to understand them."
She tilts her head to one side and smiles. An artist who wanted to paint the trueCotillardwould find himself using colours like dusty rose or aubergine. There's an almost palpable warmth emanating from her, but it's not coupled with the usual empty cheerfulness found in many Hollywood stars.
"Look, if you work all the time, if just do movie after movie, you're playing another person more than you're being yourself and the real you can get lost that way.
"If you don't live your real life, then you become empty and the only place you can find authentic inspiration is in reality. That's where the truth is."
To play Luisa, she went for her inspiration to a pair of real directors' wives.
Because Nine is based on Fellini's autobiographical 8 1/2, it's always been understood that the long-suffering wife who gave up her acting career for a while was inspired by Fellini's spouse,Giulietta Masina.
"Yes, part of my inspiration was her," says Cotillard. "If you look at the films she made for Fellini in their early days together, you can see a great artist who was also a very fragile woman.
"But I found even more material in the documentary film called Hearts of Darkness, about (Francis Ford) Coppola shooting Apocalypse Now. His wife Eleanor had to endure the most blatant infidelities on the set, much like Luisa does with Guido, but yet she remained so dedicated to him until he finally pushed her too far. Just like Guido does with Luisa."
Cotillard has slipped into a world where the real Giulietta and Eleanor have taken equal footing with the fictional Luisa and you begin to see how her art is formed.
"When you understand someone, you can love them no matter how terrible they seem to be. She knows that Guido, as an artist, as a director, lives in a world where his desires are all fulfilled. I want this set, I want this costume..." She stops, with a catch in her throat. "I want this actress. So you take them all.
"Luisa can deal with the physical infidelity, even though it is painful to her. What she finds unforgivable is the lie."
She slips into one of her songs from the film, "My Husband Makes Movies," whispering a few lines softly in her endearingly husky tremolo.
"Singing with Guido all night on the phone ... long ago, someone else ago."
Then she looks up suddenly and Luisa has vanished.
"Rob (Marshall) told me he wanted to make people feel Guido ought to be with Luisa again. That was my job. Where do I find it all? In my character. Never in myself."
Her voice rises again. "I will never, never use something from my own life. I feel it is too dangerous.
"If you wake up an old pain, for example, because you have to cry in a scene, then how do you put that pain back to sleep again?"
But where did Cotillard find the depths of degradation she needed to allow a room full of men to paw her as she stripped in front of her husband in the fantasy song "Take It All"?
"I found it in Luisa. She carried me through it. It's such a gift for an actress to find a character who will enable her to free herself that completely.
"You can't find it in every part you play, but when you do, you say `Yes, I am free! Thank you, my friend!'"