Cate Blanchett talks to Steve Pratt about one of the most difficult - and embarrassing - roles she's had to play in her career - that of a teacher who has an affair with one of her under-age pupils.

Actors often tell you that filming screen sex scenes - performing an intimate act in front of dozens of people and a movie camera - are embarrassing and difficult to do. Oscar-winning Cate Blanchett is no exception, although she had an additional worry shooting such scenes for her new movie, Notes On A Scandal.

For she plays art teacher Sheba who has a secret affair with one of her under-age pupils. And although 16-year-old newcomer Andrew Simpson, who plays her teenage lover, is over the age of consent, she was still apprehensive.

She brushes aside director Richard Eyre's comment that "they approached it as a professional task and the choreography of it was kind of surgical". Blanchett says it was "a complete veneer", adding: "I'm glad it was dark because I completely blushed my way through the whole thing."

Her secret affair with student Steven shatters the world of married-with-children art teacher Sheba, putting her at the mercy of an embittered, nasty fellow teacher (played by Judi Dench) intent on using the situation to her own advantage.

"I think the casting process was really interesting, maybe this is my morality coming in again but it was important to me that the actor was above the age of consent. Although really, what's the difference between 15 and 16. It's the law, yeah, but he is very mature.

"It wasn't really until the end of shooting that I sort of gasped. He wrote me this handwritten letter that made me want to weep, about what the film had meant to him. It was then that I thought he was so young. You just tend to treat all actors like normal actors once they're there."

Eyre made them feel very at ease in filming the love scenes, she says. "There was an odd discussion about how much of his buttocks we could see, that was a bit awkward. But irrespective of whether the actor is 15 or 57, there's always that discussion of how much you can actually see."

Australian-born Blanchett first came to the attention of cinema audiences a decade ago in Oscar And Lucinda, opposite Ralph Fiennes. Her choices since then have reflected a refusal to be tied to any type or nationality.

She won her best supporting actress Oscar for portraying Katherine Hepburn in The Aviator. Previously, she received another nomination for her portrait of Queen Elizabeth I in the historical drama Elizabeth, which won her awards around the world. This year she'll be reunited with director Shekhar Kapur in The Golden Age, featuring further episodes from the life of the Virgin Queen.

She's an actress as likely to turn up as an Irish journalist in Veronica Guerin as the ethereal Galadriel, Queen of the Elves, in The Lord Of The Rings trilogy. She must also be one of the few capable of holding their own when the formidable Judi Dench is in barnstorming mode, as she is in Notes On A Scandal, which is adapted by Patrick Marber from Zoe Heller's Booker Prize-nominated novel.

The pair's on-screen relationship takes a violent turn when Sheba's affair becomes public knowledge and the two women fight. Blanchett sounds as worried about filming this scene as the sexual encounters.

"Both of us were dreading it, to be honest, because it's about finding the pitch of a scene like that," she confesses. "The stakes, and the expression of those stakes are so high, but it's also absurd, the things that they're saying to one another.

"What Patrick had written gave the scene a buoyancy which was actually, in the end, quite fun to play. But we did down a bottle of champagne after we'd finished it."

Blanchett has been quoted as saying that Sheba was one of the most difficult characters she'd ever had to connect to. "We always say that, don't we? It's such hard work, to dredge it up," she says.

"I'm not interested in playing characters who see the world through my prism. I think the journey of understanding any character is to see how they tick and how they differ from you. Probably the hardest thing was to liberate her from my own morality.

"I was quite shocked at the tone I took, the judgments I had of the relationship she embarked on, you know, sex with a minor. But it's the stuff of great drama.

"It's a fascinating journey to play - someone who's quite icy and gossamer and coy in the beginning, who then ends up being thrust out of a basement flat, screaming in her pyjamas, dressed as Siouxsie and the Banshees, going after the paparazzi."

She may mention an actor's favourite word - journey - but won't be drawn into detailing how she builds a character, such as watching people and using their mannerisms in performances. "As an actor I suppose you're constantly observing," she says. "I don't sit in restaurants making notes, I don't live my life in order to then feed it into my work. But I guess, by osmosis, you observe these things. And when you have a character as richly drawn I suppose then there are subconscious mental notes that you've made.

"She's a type of woman, yet I don't find that she's a cliche - at least I hope she's not."

She would like to think that her Oscar win has changed her career, perhaps seen her name added to different lists. She hopes so but knows that winning these things can be "a bit of a curse, depending on who you are and how you think", she says.

"But I haven't been on a journey to get anywhere in particular, so that hasn't changed. And my criteria for choosing projects hasn't changed. "Perhaps it's changed by being a parent in that you ask yourself how long you'll be away from home. My eldest child is approaching school age so that becomes different, they're less portable, so that's probably shifted things."

She and her husband, screenwriter Andrew Upton, will be quitting these shores, at least temporarily, when they become co-artistic directors of the Sydney Theatre Company. She plans to combine those duties with acting, although inevitably it will leave less time for movie-making.

"You're peripatetic when you work in this industry. But as long as the film industry will have me, I will have it. If Martin Scorsese calls tomorrow...," she says.

They assume their artistic director roles next year, although the first season planned by them won't be on stage until 2009. She calls it an enormous responsibility, but it must be a risk, too, to "disappear" to the other side of the world for part of the year to do theatre. But then Blanchett has never played it safe.

Notes On A Scandal (15) is showing in cinemas now.