It's just after 2.30pm, Paris time, when Cate Blanchett arrives for our interview at Le Bristol hotel, one of the city's more luxurious five-star dwellings.

The Australian actor bursts into the room like an old friend with a crash-bang "Hello!" greeting, before sitting down at the table. She glances at my recorder. "If it doesn't work you can just make it up," she says. A fine example of just how unprecious she is, and it might also be a sneaky way of showing that she has no interest in reading what's written about her.

Then again, Blanchett is the sort of star where newspaper profiles are almost entirely constructed from superlatives - due to a near-peerless career. The roster of directors who've hired her - Steven Spielberg, David Fincher, Anthony Minghella, Wes Anderson, Peter Jackson, to name but five - read like a modern movie masterclass. As for her male co-stars - Brad Pitt, George Clooney, Leonardo DiCaprio, Russell Crowe, Matt Damon, to name another five - it's surely enough to make her the envy of every woman on the planet.

Despite this, Blanchett, 44, is not the sort of actor who trades greatly on her looks. Yes, she's the face of Giorgio Armani's new fragrance Si in a deal said to be worth $10 million (and given that flawless skin of hers, you'd have to say it was money well spent). But she'd rather be on stage, sweating blood for a part, than sauntering up a red carpet to a blaze of flashbulbs. Maybe this is what comes of being married for 16 years - to playwright Andrew Upton - and being a mother-of-three, but Blanchett is all about the graft.

In her time, she's been nominated for five Oscars, and won a Best Supporting statue for her take on Katharine Hepburn in Martin Scorsese's The Aviator. She's played murdered Irish journalist Veronica Guerin, Elizabeth I of England (twice) and even Bob Dylan in Todd Haynes's I'm Not There. On stage, she was Richard II and clearly has no problem tapping into her masculine side. "I always dressed as a man when I was at school," she says. "I loved wearing a tie and a shirt, and I was always wearing suits. Annie Lennox was my hero. I was always playing men in high school."

Today, as she pads into the room barefoot, she's wearing androgynous black - long trousers, a short-sleeved top and a tailored waistcoat that clings to her slender figure. Radiating a casual, effortless glamour, her "eclectic" dress sense comes with a helping hand, though not from the army of stylists most A-list stars require to help them accessorise. "When I have my moments of insomnia, you'll find me on," she says. "The internet is very handy, isn't it?"

Despite that, you get the feeling that fashion is fairly low on her list of priorities - even though next day she must slide into a "very small dress" for the Paris premiere of her latest film, Blue Jasmine. Her first movie for Woody Allen, it's already proved one of the sleeper hits of the summer, with a $25m US box office. Critics have raved, the New York Times calling it Allen's "most sustained, satisfying and resonant film since Match Point". You could go back far further than this 2005 high-point; Blue Jasmine might just be the best thing he's done since 1989's Crimes And Misdemeanours.

Put this down, partly at least, to Blanchett, who quite brilliantly brings the character of Jasmine French to life. A well-to-do Park Avenue princess, Jasmine sees her elite Manhattan universe crumble when she discovers her husband (played by Alec Baldwin) has been both unfaithful and involved in financial misdeeds.

Forced to move to San Francisco to live with her less affluent sister Ginger (Sally Hawkins) and - horror of horrors - go to work, Jasmine gradually slips towards an alcohol-and-pills-fuelled breakdown.

It's a masterful portrayal by Blanchett. Painful, tragic, self-mocking, this delusional creature claws for our sympathies (and sometimes gets them) in a way that recalls Blanche DuBois - a role Blanchett has played on stage - in Tennessee Williams' A Streetcar Named Desire. Little wonder the New Yorker said Blanchett "gives the most complicated and demanding performance of her career". Modesty aside, Blanchett concurs: "That was the challenging thing to play. Jasmine is in and out … depending on the cocktail of drugs and alcohol she's on at any given moment."

For Blanchett, it meant looking into mental illness. "A very close friend of mine, his brother has dementia and he's quite a young man. And he said [what is] most terrifying and moving … is that he's clinging to sanity. And I think that's what you watch … In a way, it's far more comfortable when they're completely mad, because you've lost them."

She went on YouTube, too, to uncover more about Jasmine's pill-of-choice, Xanax, used to treat anxiety and panic disorders. "I'd never had a Xanax in my life, thank goodness. But the amount of people on YouTube, either on Xanax or talking about the effects of being on Xanax … it was really important for me to know what that was. Sometimes, you think 'I'll be sweaty in this scene.' Obviously, Jasmine is having a panic attack, even though it's never mentioned. So I found that fascinating, shocking and heartbreaking, to mine that furrow."

Remarkably, Blanchett did not discreetly divorce herself from her domestic duties during this period. She and Upton have three boys - Dashiell, 12; Roman, nine; Ignatius, five - and all were with her during the shoot in San Francisco, Allen's first film set there since 1969's Take The Money And Run. "They're not interested whether you're playing Lady Macbeth or Jasmine French or Hamlet. They just want their dinner and for you to do their homework with them." While this gave her some sense of normality, the role brought other problems. "It affects your dream-life. I didn't sleep particularly well."

Every day on set, she reveals, she'd tell herself: don't screw it up. Does this mean, underneath this placid persona that Blanchett projects, she's a bundle of anxieties? "That wasn't born out of anxiety, that was just pragmatic," she answers. "You've been given a really great opportunity - it's my job to make it jump off the page. Not to make it less than Woody's offering. But I say that to myself every time. I'm not particularly needy and I'm not particularly anxious. I don't look for a director to tell me I'm doing a good job, or that I'm great. I don't need to be stroked. It's more my own yardstick."

This is just as well, as Allen is not the kind of director to hold his charges' hands. The first time Allen saw her, he says, was in Anthony Minghella's 1999 drama The Talented Mr Ripley. "She was amazing," he says. "I've seen her over the years and she's always great."

He puts her in a rarefied group - those who achieve the leap from being good actors to great ones. "Cate has that depth and complexity. Meryl Streep has it. And I've worked with some actresses that have had it - Geraldine Page had it, Gena Rowlands, Dianne Wiest, a number of them. And Cate is the one of her age group now who has that amazing gift."

Quite how Blanchett does it is another thing. Juggling parenthood and a film career is one thing but since early 2008 she and her husband have been co-creative directors of the Sydney Theatre Company. Blanchett stepped down this January, relinquishing control to Upton, although when we meet in late August, she's just come to the end of a two-month run in Jean Genet's 1947 play The Maids, alongside Isabelle Huppert - again to reviews full of those superlatives.

If it meant we lost Blanchett on screen for a time - bar her fun-but-forgettable appearances in Ridley Scott's Robin Hood and Spielberg's last Indiana Jones film - her stage work has aided her development as an actor immeasurably. "Just to be on stage with that level of intensity for that number of years was fantastic," she says. "I'd be an idiot if I hadn't got better. It sounds obvious but it's true - you're very intimately connected with your audience. It makes your sense of …" She pauses, thoughts ticking over, her blue eyes flickering. "I think you become more fearless."

It'd be wrong to suggest Blanchett has been this gutsy from the start. Blanchett is the middle of three children to Robert, a naval officer from Texas who met her mother, June, in Melbourne. He stayed and became an advertising executive, and Blanchett and older brother Bob (now a computer systems engineer) and sister Genevieve (a theatrical designer) were born. When she hit 10, her father died of a heart attack, leaving her mother to raise the family alone.

She once claimed she had "no memories" of her childhood - and this sudden tragedy may explain why. But Blanchett is not a closed book on the subject of her upbringing; little details spill out from time to time. When we touch of the subject of The Missing, the Ron Howard Western she starred in, she paints a rather homely domestic picture, related to her father's Texan origins. "I grew up hearing John Wayne films bouncing and shooting away on a Sunday afternoon."

Famously she once described herself as "part extrovert, part wallflower" growing up. Attending Ivanhoe Girls' Grammar School and then Melbourne Methodist Ladies' College, aside from her penchant for dressing like a man, her rebellion was textbook teenage. "I went through a big goth [phase] and then a punk period - I even shaved my head," she smiles. She left the University of Melbourne, where she was studying the eminently sensible economics and fine arts, after just one year.

She did what most Australians do, and backpacked her way out of the country. One pit-stop was Egypt, where she met a Scottish casting director who was recruiting American-looking actors for an Egyptian boxing movie called Kaboria. In need of funds, Blanchett jumped at it. "They were going to pay two Egyptian pounds that was going to pay my rent for the week so I said, 'Sure.'" Given pom-poms, and told to cheer on the main actor, Blanchett reports that the experience was both "boring" and "embarrassing".

But something must have clicked. Back in Australia, she moved to Sydney to study at the National Institute of Dramatic Art, graduating in 1992. She can still remember days in her dimly-lit bedsit, where she had to watch the pennies so much she could only afford to buy one cup of coffee every other day. But almost straight out of drama school, her first job was in David Mamet's Oleanna for the Sydney Theatre Company opposite Geoffrey Rush. The Sydney Theatre Critics' gave her their Best Newcomer Award.

Director Gillian Armstrong caught the performance and, three years later, when it came to casting her adaptation of Peter Carey's novel Oscar And Lucinda, thought of Blanchett. Determined to succeed, she learned to play poker for the role - which was just as well, as it was over a game that she and Upton finally got together. Until then, she reports, he thought she was aloof and she thought he arrogant. Then came the poker match. "He was in the middle of telling how he was attracted to a friend of mine and then we suddenly kissed." Three weeks later, he asked her to marry him.

Wed in 1997, they stayed in London for a time because Upton's agent was based there and Blanchett was on stage in a production of David Hare's Plenty at the Almeida. Moreover, it kept her in the UK and in the frame for Shekhar Kapur's 1998 period bio Elizabeth, her breakthrough role. Playing the flame-haired monarch - a part she repeated for the sequel, 2007's Elizabeth: The Golden Age - she won a Bafta and the first of her Oscar nominations, with a performance of steely resolve that even saw her study Elizabeth I's handwriting.

While Kapur's film established her movie credentials, Blanchett has never let the stage disappear from her CV - as her five-year tenure at the Sydney Theatre Company attests. That said, with that now at an end, Blanchett is readying herself for a rash of movie appearances. With directors presumably on alert that Blanchett was ready to emerge from her theatrical cocoon, she's already lined up work with David Mamet (JFK thriller Blackbird), Kenneth Branagh (Cinderella - in which, deliciously, she'll play the evil stepmother) and a reunion with Todd Haynes (returning her to the work of Ripley creator Patricia Highsmith, in an adaptation of her story, The Price Of Salt).

Already, she's wrapped not one but two films for Terrence Malick, the once-reclusive Badlands director who has suddenly discovered a love of working. Both are still under wraps - one is called Knight Of Cups, one remains untitled - and both co-star Christian Bale and Natalie Portman. Even if she wanted to, Blanchett is unable to explain her roles, given the freewheeling way the director works (he's renowned for trimming away whole performances from the end product). "I don't know what my part in the ultimate narrative will be, but that's the contract you enter into with Mr Malick."

Before that, however, is a return to Middle Earth in the second part of Peter Jackson's adaptation of JRR Tolkien's The Hobbit, reprising the role of ethereal elf Galadriel that she played in Lord Of The Rings, despite the fact her character doesn't appear in the book. "I think Peter and Fran [Walsh, his partner and co-writer] looked at The Hobbit and thought: 'Hmm, not very many girls in this. We better get a chick in there! We'll get that elf with the long hair in!'"

For an actor who has never been particularly on intimate terms with Hollywood studio movies, she seems forever in demand. "In my career, I thought I've never wanted to get anywhere in particular. I just wanted to work with interesting people on interesting projects." In January, she can be seen adding her feminine charms to The Monuments Men, a true-life Second World War-set "boy's own adventure", as she puts it, that reunites her with George Clooney, her co-star from The Good German, and Matt Damon, from Ripley.

There's even talk about Blanchett producing or directing a film. "Having now left the company, it's something Andrew and I have been thinking about," she says. But right now, the prospect of raising three children overshadows all else. Did motherhood change her? "I hope so. Before I did, people talked about it as: 'Your life will change,' like it was this heavy thing. But I found it utterly expansive." She looks mildly panic-stricken for a moment. "I hope I've evolved. I hope I'm wiser. I hope I'm a better actor." There's no question about that. n

Blue Jasmine (12A) opens on September 27.