Technical run-throughs are laborious, long-winded and actors hate them. Not Blanchett. "I love the freedom of the tech. It actually liberates you, you feel unwatched. I'm really looking forward to it," she cries, greedily. It's hardly surprising that she's relishing the prospect of having a couple of days to experiment with the role.
The play in question is Plenty, written in 1978 and unquestionably one of David Hare's two or three finest, and her character Susan Traherne - originally written for Hare's then muse, Kate Nelligan - barely leaves the stage. Bar a three-week run in an Australian play at the Croydon Warehouse a few years back, Blanchett is an unknown quantity in terms of the British stage. Casting her represents a leap of faith by the Almeida's director, Jonathan Kent, but then that's what this 29-year-old actress inspires.
The film director Gillian Armstrong had been searching for six years for an actress to play the lead in Oscar and Lucinda - Blanchett did a screen test and Armstrong stopped looking. Anthony Minghella saw the finished movie and immediately cast her in his film The Talented Mr Ripley. The character has just two scenes at either end of the picture, but he knew he wanted her. Temporarily nonplussed by being asked exactly what it was about her that so impressed him, Minghella casts about for a reason. "It sounds ridiculous, but it's quite simple: if you have the chance to work with talent like that, you do."
If you add in the news that there are other films in the can, including Pushing Tin, Mike Newell's tale of rival air-traffic controllers, in which she does white-trash comedy in tight trousers, big hair and a push- up bra as Mrs John Cusack, and the headlines are pretty clear: Cate Blanchett has arrived.
Confirmation came (if it were needed) on Sunday night when, after losing out to Gwyneth Paltrow at the Oscars and the Golden Globes, she finally received her just deserts with Bafta voting her Best Actress for Elizabeth. Other rising stars in her position would have headed off for poolside Hollywood, which might lead some people - possibly including her agent - to wonder what on earth she's doing picking up a company wage in a play in London.
"I never thought that film was somehow 'making it'. Film-making can sometimes be a little clean, clinical. I like the church hall feel of rehearsals, getting my hands dirty. The other day, on my way to rehearsals, I walked past Sadler's Wells and watched the trucks unloading the scenery through the dock doors. The atmosphere that comes out at you is magical."
This is not the contrived gushing of an actress suddenly determined to grab the kudos of the theatre. She was on stage for 18 months playing Nina in The Seagull at Belvoir Street theatre, home to Company B, Sydney's most exciting theatre company. Even at drama school, she made waves, her bold choices supported by a self-possession borne out of a solid technique.
Watching her in Elizabeth, the shock is how instantaneously you believe this Australian actress to be Tudor royalty. Meeting her in the flesh, she seems slighter, with an almost fleeting sense of presence. And then she opens her mouth. It's not what she says that's surprising, it's the way she says it. The voice is low and calm, offsetting the fragility of her complexion. Light streams in through the window catching the shifting planes of her face. Braque would have envied the lines of her cheekbones and jawline, but there's nothing cold or chiselled about her look. It's merely that there's an elegance about her which comes from quiet self- possession, which is what makes her so mesmerising on screen.
Even when watching her in close-up, she seems taller than she is because her centre of gravity is so low. It's all about relaxation. All of which comes in useful when playing high-status roles such as the Queen of England or Lady Chiltern in Oscar Wilde's An Ideal Husband, her latest film, which is released on Friday. Most actors, when asked about their choice of roles, will tell you what a great part or opportunity it was, or how much they wanted to work with so-and-so. Blanchett's reason for doing this film? "I was really interested to see if such heightened dialogue could be played naturalistically."
Nothing is more illuminating about both the qualities and the quality of a screen actor than watching her or him "think". Put a weak actor in front of the lens and they'll display puzzlement with, say, a furrowed brow, confusion with a tilt of the head, doubt with a widening of the eyes. If the edits surrounding such displays are strong enough, this empty acting style can prove effective.
However, watching Blanchett play the complex comedy at the climax of An Ideal Husband is something else altogether. She's the focus of a scene in which everyone brings in fresh revelations, forcing her to revise her opinions and think very fast. But instead of opting for a simple external approach, she takes the much more exciting (and exacting) choice of presenting an impassive exterior while managing to convey Lady Mabel's inner turmoil as she is forced to compromise her ideals and lie to her husband. No wonder the camera holds her in tight close-up, her eyes darting about in disguised terror. This is not "thinking" acting: Blanchett is responding to what is happening inside her character.
Actors are often castigated for supposedly airy talk about being "in the moment", but that's precisely where Blanchett's characters live. You never get the sense that she's trying to impress by showing off how she can achieve an effect. In career terms, she's disgracefully clear-headed about the thorny issue of reading (and believing) your own press. Does she read the ever increasing mountain of press coverage? She sighs. "I try not to... but as someone said to me, 'trying is lying'. Of course, it's fantastic if you get great reviews but they're like lotteries: they're great if you win and a terrible sham if you lose. People tell me not to be self-deprecating but I think I have to gloss over the good in the same way that I have to gloss over the bad.
"The great thing about Plenty is that there is a genuine amount of trust within the company. We're all trying to do something together," she insists, observing that the play is as much about her husband's character - played by Julian Wadham - as it is about her own. "Whether we succeed or not, we can't tell. You can't plan for success, of course, but equally you also don't want to plan for failure by not taking risks and not opening up things which you worry you might fall flat on your face for doing." She beams. "That void you feel opening up beneath you... I find that quite exciting."
She apologises - unnecessarily - for being inarticulate, but the only point on which she's truly vague is the future. Her agent is bemused by the fact that she's barely half-way through a hot script she's had for more than over two weeks. "It takes me so long to decide. I need time to digest things. There's a lot of stuff out there, I guess, but I find it too disappointing when I feel I'm in things for the wrong reasons. Luckily it's only happened to me once or twice. So I'd rather wait."
In the meantime, she's done a run-through of Plenty in front of David Hare, which makes the press night seem less daunting. Her only real frustrations at the moment are the expectations of the world from which she's made a temporary escape. "I get very frustrated with the fact that in film people are still very surprised if women my age can do something other than what they look like they can do. I knew after The Seagull that I really wanted to do something where people can demand a lot of me, push me beyond where I think I can go. I love being directed. I don't take it personally if someone says, 'Oh, I didn't like that'. It makes you search more. Maybe that's why I continue to do this."
'Plenty' previews at the Albery Theatre (0171-369 1730) from tomorrow; 'An Ideal Husband' opens on Friday