Siân Brooke is trying very hard not to click her heels together, thrice, while mumbling anything about her native Lichfield. At 28, she’s about to find herself on the biggest stage of her career, in the world’s most iconic pair of slippers. They belong, of course, to Dorothy. Buffy staked transient claim to the title, but Dorothy has long been the schoolgirl supernatural slayer of choice (using such handy, common or garden objects as (a) a house and (b) a bucket of water. Ne’er an ancient hieroglyph or a bespectacled Brit in sight) and has proved the yellow brick road of fame to the Oz of undisputed divadom for more than one Hollywood starlet.

Brooke was somewhat bemused when she got the call to audition. Jude Kelly announced in May that she would be directing John Kane’s adaptation of the classic 1939 movie at the Festival Hall, the first full-scale revival of The Wizard of Oz in London for 20 years. At the time, Brooke was playing the harassed Lulu in David Farr’s triumphant reprisal of Pinter’s The Birthday Party at the Royal Court. “I thought, ‘That’s weird,’ ” she says, as she lunches at a Southwark café, “I haven’t really done any musicals. ‘Why me?’ ”

Since leaving RADA in 2002, Brooke has notched up a bewildering array of heavyweight roles. One of her first jobs was with the legendary Franco Zeffirelli, a Pirandello revival with Joan Plowright in the West End. A season at the RSC followed, most notably Cordelia in Bill Alexander’s King Lear, and Juliet to Matthew Rhys’s Romeo in Peter Gill’s production. In between, there were two new plays at the Royal Court, and a comic turn in Richard Bean’s epic, award-winning Harvest at the Hampstead theatre. On TV, All About George in 2005 and Cape Wrath in 2007 won further acclaim.

Not, though, the kind of acclaim likely to land her the lead in a family musical at the Festival Hall. How was Kelly to know that she would look fetching in gingham, earn the love of a small dog, and be able to warble her way over a rainbow in the towering shadow of Judy Garland?

“I’ve no idea,” Brooke says. “But I did a lot of musicals when I was young,” in the National Youth Theatre and at school in the Midlands. “I actually played Dorothy when I was 13, so in a very strange way this takes me right back to my beginnings. I’ve come full circle.”

Brooke’s concerns about the part are few. She is slightly worried she might be upstaged by Bobby, the West Highland terrier who will be playing the pivotal part of Toto. And there’s a vague fear over whether she will be able to breathe. Like the 16-year-old Garland, she will be heavily strapped to squash her bust to prepubescent proportions. In Frank L. Baum’s 1900 novel, Dorothy was 13.

She was also blonde. Originally, Shirley Temple was signed to the MGM film, with Garland drafted in at the last minute and handed a pasteboard, hay-coloured wig. When Victor Fleming became director, he advised Garland to “go natural”, and Dorothy became a brunette.

In deference, Brooke will be covering her blonde locks with a chestnut wig, but she’s not overly troubled about the inevitable comparisons. “I could never be Judy Garland. I can only be myself. People asked me with Romeo and Juliet, ‘How will you say those lines?’ And it’s the same with this, I’ve got to approach it as though it’s never been done before, and just let the words come out. I think she’ll be more of a tomboy. I’m not going to come on in dungarees with a spanner or anything, but I want a bit of that ‘Come on then, why not?’ quality. We’re not playing it like a panto, Jude’s very clear that Dorothy’s completely real.”

Kelly has come under fire in some quarters for staging what anyway feels like a Christmas show in the Festival Hall. It lacks wings and fly space above the stage area, which hampers the movement of both actors and possible effects. It’s also a stroll from the £7 million production of Wicked in the West End, the hit musical that tells the witches’ tale.

Brooke retorts: “You can’t restrict these things, and I don’t think you should. To have a full orchestra, in the acoustics of that space, performing this amazing score is great.”

Kelly, who last directed the musical at the West Yorkshire Playhouse, will bring a cast of 50 to the stage, including Gary Wilmot as the cowardly lion. As for how they will pull off a tornado, flying monkeys, melting witches and the like, “Wait and see,” Brooke says. “Pure, imaginative stage-craft. This really is a family show; people come to it at all ages because there are so many levels to it.”

Initially, Dorothy’s journey from sepia Kansas to shimmering Oz was read as a metaphor for the medium of film itself, as the static cams of black and white silent movies gave way to blazing Technicolor talkies. In the Sixties, many insisted that Dorothy’s encounter with the twister was a thinly veiled allusion to an LSD trip. It has also been interpreted as a coming-out story, in which the restrictive rules of small-town Kansas are vanquished with the aid of a fine pair of heels and a benevolent fairy.

For Brooke, it’s a perfect description of her experience of acting. She says: “When I did this at 13, I felt a bit like Dorothy; all the magic and colour, getting swept up in the world of the play. I just loved it. I knew then I wanted to be an actor.”

At the time, old-fashioned pluck and determination were the only advantages at her disposal. She was the youngest of three children and no one in Brooke’s family is in the business. Her mother is a teacher, her father a policeman. To help to fund her RADA training, she borrowed her school’s stage to host a variety night, her parents wrote “thousands” of letters to famous folks from Who’s Who, asking for help, and her mum devised a sponsored plastic duck race. “We dropped 300 of them off a bridge into the river and ran down to see who’d won.”

But her first year at RADA proved challenging. “I was overwhelmed,” Brooke says, “I didn’t know anyone in London. There were these 30 kids, very strong, confident characters who had been to university, which I hadn’t. I got quite ill in my first year; I just didn’t feel like me.” She ended up taking a year out, going home to recuperate, and rejoining the year below. “From that moment on,” she grins, “I had the best time. It was like a light bulb went on.”

It hasn’t flickered since. She now lives in South London with her boyfriend, the actor/director Bill Buckhurst, and their own West Highland terrier, Monty. She has landed an auspicious string of parts, Dorothy being perhaps the most portentous. It’s not just Garland who has tried on the ruby slippers; so too has Diana Ross, in the 1978 screen adaptation of the all-black Broadway musical The Wiz. Denise Van Outen had a go in 1995 (a regrettable TV incident on Channel Five); also Jewel and Ashanti.

It is unlikely that Brooke will follow Garland and Ross’s leads to post-Dorothy drug addiction. She practises yoga before performing, and deals with stage fright through pranic breathing. “But if you see me, a year from now, all jittery, singing Over the Rainbow in a corner somewhere, you will know it’s happened.”

The Wizard of Oz, Festival Hall, London SE1 (www.southbankcentre.co.uk 0871 6632500), July 23-Aug 31 2008

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