However the spin-doctors play it, there is only one reason why: Mel Gibson. A virtual Hollywood pariah for a series of inflammatory remarks over the past three years, Gibson is the film's millstone.

Having previously co-starred with him in 1994 western Maverick, Foster has been good friends with Gibson ever since. And you can't argue with her logic in casting him. His performances have frequently verged on insanity (from Mad Max to Lethal Weapon) and he's tailor-made for the role of Walter Black. A suicidal father-of-two, Black's radical solution to overcoming depression is puppet therapy. In a moment of madness, he slips on a beaver hand puppet and begins to let the cuddly creature do his talking - a bizarre act of ventriloquism that revitalises his relationship with his estranged wife (played by Foster).

However suited he is to the role, Foster is left with the difficult task of defending Gibson's utterances when others have shunned him. 'I can't excuse Mel's behaviour. Only he can explain that. We're all responsible for our own behaviour,' she says. 'But I do know the man that I know. He's been a friend for many, many years. As a friend, he is kind and loyal and thoughtful and I can spend hours on the phone with him, talking about life. And he's complex - and I appreciate his complexity.'

Does she think the film will rehabilitate him? She pauses, looking down at the table. 'I don't know. I have no idea. I really don't have any idea about that. I know he's incredibly proud of the movie, and proud of what he's shown,' she says. 'He wants people to see that side of him, and he's an incredibly private man. What you see on screen is as deep as you could possibly get. I think he understood the complexity of the character and really understood a film about a man who's suffering, and who is struggling with wanting to change and wanting to be different than he is - the self-hatred of it.'

Gibson aside, Foster clearly has her own reasons for being drawn to the script. 'I've known a lot of people who were depressed, and there was a lot of depression in my family,' she says. 'I think the film explores it without looking at it like a disease movie.'

As she later notes, 'All the movies that I make in some ways have to be the story of my life.' Looking back, Home for the Holidays, a comedy set around a nightmarish family Thanksgiving, 'was about being in your 30s' - and made when Foster was 33. As for her 1992 directorial debut Little Man Tate, it was a warm, intimate film about a child prodigy's relationship with his working-class mother.

Born in Los Angeles, Foster knows exactly this world. Cast as the face of suntan lotion Coppertone when she was just three years old, she made nearly 40 commercials in the next five years alone. With her mother Evelyn, who worked for a film producer, driving her career, Foster went from ads to television (parts in Bonanza, The Partridge Family and Ironside) in an eye-blink. But she refuses to see herself like the gifted boy in Little Man Tate.

'I had a prodigious life, living in a grown-up world when I was a child. But I think my abilities were about perceptiveness. That's very different to all those really cool prodigies that can play piano.'

Film followed - most famously playing a teenage prostitute in Taxi Driver, which saw the then 14-year-old receiving the first of four Oscar nominations. Her next two nods - for rape drama The Accused and hit horror The Silence of the Lambs - came when she was an adult. Both times she claimed the best actress award, further cementing her reputation as someone who favours playing gutsy women - a trend that has continued into the past decade, in films such as Panic Room and The Brave One.

Yet Foster is not quite so cast-iron. 'I make movies about people in spiritual crisis,' she says, at one point, 'because it's a way for me to spend the time, the energy, the focus and the obsession to come to terms with my own spiritual crises.'

Quite what these are, Foster won't say - although the constant tabloid speculation about her private life might well be part of it. Four years ago, in an acceptance speech at a Women in Entertainment bash, she famously praised one Cydney Bernard 'who sticks with me through all the rotten and the bliss'. They met on Sommersby in 1993 and were said to be partners for the following 14 years, during which time Foster gave birth to Charles, now 12, and Kit, nine, with the father remaining anonymous. But Foster had never publicly acknowledged their relationship, at least until her emotional speech, nor commented in any way on her sexuality.

Since then, she was said to have split from Bernard, hooking up - for a year at least - with screenwriter Cynthia Mort. There have been other traumas - not least the horrific time she had with the collapse of circus-set movie Flora Plum, which she was set to direct until her leading man, Russell Crowe, injured his shoulder. 'I always feel like I made that movie already, because I got it set up another two times after that,' she says, almost laughing at her misfortune. And now, there's the ill-fated reception of The Beaver, over which Foster remains bullish. 'It exists on DVD. It walks and talks the way it walks and talks, and that's something that nobody can take away.'

Foster has recently ploughed her energies back into acting, co-starring with Kate Winslet in Roman Polanski's forthcoming Carnage. And in August, she starts work on Elysium, a new sci-fi thriller from Neill Blomkamp, the director of District 9. By her standards, it's a prolific period. 'I think I will always act,' she says. 'I can't escape it really. I can't imagine I would stop acting after 45 years.'

For her, it offers a 'completely solitary experience' that she lives for. 'It belongs to no one else but me, and no one else can take it away.' Like Walter's puppet, this is her survival tool.

The Beaver opens on July 28