Was the show going to be popular?

Would Broadway audiences buy into a musical makeover of the much-loved Wizard Of Oz scenario? How would they react when it was revealed that the whizz-of-a-wiz wasn't really Mr Nice Guy and that the Wicked Witch of the West wasn't necessarily an evil meanie who would kill a little girl for a pair of shoes?

A decade on, and those nervous questions have been answered. Wicked isn't just popular, it's a global phenomenon with over 4000 performances chalked up on Broadway - making it the 11th longest-running show on New York's legendary Great White Way - and various touring cadres staging the production across the world, from Japan to Mexico, Australia to London (where it's been playing solidly since 2006). Some 39 million people have watched as a familiar story has taken some surprising, even unexpected turns; whether or not their attitudes to L Frank Baum's original characters have changed for the better, they've probably been changed for good.

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And now Wicked is coming to Glasgow. Flying monkeys, the Emerald City, the whole kit and caboodle - including Glinda's bubble and Elphaba's broom - are on-stage at the King's for the show's Scottish Premiere next week. So far the company's UK tour has been a triumph. But already the two leading ladies, Nikki Davis-Jones (the green-faced witch, Elphaba) and Emily Tierney ('good witch' Glinda) know that this time, there's going to be a bricks'n'mortar challenge. Something they haven't experienced before.

"The stage has got a rake on it," says Tierney. She laughs, but serious steps have been taken to ensure everyone can cope with the tilt that inclines from back-cloth to edge of the stage.

"Our physiotherapist has been giving us these exercises," she continues, "just so as we can adjust to the slant. I'm totally rubbish at rakes - I have this nightmare vision of Glinda, in her huge pink dress, stepping out of her bubble and just rolling down into the orchestra pit ... Would she still come up singing? Oh probably. I think that's why audiences love Glinda. She's such an optimist. She's really too naive to think the worst of anybody, she's not even sure that bad things do happen. But by the end, she's lost the person she loves, lost her best-best friend and she's had to question her own morals and behaviour.

"It's a huge arc for any character to travel, but that's why it's such a gift of a part. You get to sing some amazing songs, really act the whole range from comedy to sad - and get to wear some unbelievable frocks. So it's worth living like a nun, doing yoga, giving up on late nights and getting seriously excited about kale or spirulina in juicing recipes!"

In the next door dressing room, Nikki Davis-Jones is getting to grips with another kind of green: the distinctive make-up that transforms her into Elphaba, the supposedly malevolent witch at the heart of Gregory Maguire's novel, a revisionist take on Baum's original story and the inspiration behind the musical Wicked.

"It's really easy to apply - but definitely not easy to get off," says Davis-Jones, going on to outline the regime attached to 'greening-up'. The colour is water-based, slips on like paint and doesn't feel heavy or mask-like. Removing it, however, is a step-by-step ritual that can't be skimped. "If I don't do it properly," she says, "within two days my skin's bad. So when I come off-stage, it's stuff to loosen the green and move it around, then water that makes it all foamy before you wipe that all off - and you're no longer bright green, you just look really ill. So you exfoliate, use a gentle calming cleanser on top of that, then a nice moisturiser. On matinee days, I admit I don't always scrub behind my ears..."

She lifts a strand of hair to give me a peek: Elphaba-green is clinging to her skin.

"I've got great skin-care products, and they organise regular facials for me, but this is absolutely my dream come true. And occasional spots? So what. You know, in The Wizard Of Oz film, the Wicked Witch is only on-screen for about 11 minutes total. Yet everyone remembers her as this villain who hangs over everything like an evil threat. As Elphaba, I can go on-stage and turn that around. Make people realise that she's really a heroine who - because of what she believes in - sacrifices herself. Decides that if she's the villain everybody wants to blame, well, so be it. And when I sing 'No good deed goes unpunished...' that's such a turning point in the story, and for me. It is an emotional moment, and it's also a very dark point that isn't maybe expected in what is a family entertainment.

"But that's one of the reasons I think this show is genius. The first time I saw it in London, I couldn't get over how clever it was. How all the familiar characters - including Lion, Tin Man and Scarecrow - are all part of these plot twists that stack up against Elphaba. And I'm thinking I'd love to be her, but I just never thought I'd be able to even audition for it. I wasn't sure my voice was strong enough, or my acting, or my physical stamina ..."

In fact, Davis-Jones never lost sight of that wishful ambition. "I worked to get it," she says with a little smile that belies years of self-discipline, classes, being understudy to various leading ladies - and yes, she was the standby Elphaba in the London production - before she got her very own broom and the chance to put her own stamp on the role, night after night on the UK tour. Cardiff, where we meet up, sees the production eight months into a schedule that extends into 2015, but there's no hint of Davis-Jones feeling jaded by how Elphaba has taken over every aspect of her life off-stage as well as on.

None of the magic has worn off for Tierney either - though there's daily maintenance on her ginormous Glinda 'bubble' frock. "They have somebody sewing on any sequins that I've lost on-stage," she says, "which I find amazing. Who's sitting in the circle counting? But the whole touring production works to the same standards as the West End show. And where something doesn't quite fit into a venue, our brilliant tech team come up with a way to get the same effect by another route. Luckily my bubble - that I fly in on as Glinda - seems to work everywhere."

It turns out that Tierney has played Glinda in stage versions of The Wizard Of Oz. "I'm just meant to be Glinda," she laughs. "But of course when I first meet Elphaba, I've no idea that we're going to become best friends, or that I will end up the Good Witch and she... well, actually, I think that's one of the ways that audiences can really connect with Wicked. Because we never get to say a proper goodbye, and that loss is what changes Glinda 'for good' - and singing that song [For Good] gets to me every night, without fail."

Davis-Jones, meanwhile, is getting ready for show time. "I always think of my grandad as I'm getting ready," she says. "When I was little, we used to sing together. He really should have been a professional, so part of me is doing this for him and in memory of how he always inspired me. He used to say to me 'Before a show, go and sit on the stage. Just five minutes on your own, settling in to it.' And I do that. I love the backstage smell, the sound of a theatre coming alive, orchestra tuning up, audience arriving. Just realising that people have made an effort to come - like the girl who did my last facial. She'd booked her seats a year ago. I want her, want everybody, to have the best experience I and the whole company can give them."

Now that's the kind of attitude that makes a show, and its stars, not just popular but memorable.

Wicked is at the King's Theatre, Glasgow from May 6-31. It returns to Scotland in the autumn, at the Playhouse, Edinburgh (Nov 19-Jan 10) and then at His Majesty's Aberdeen (May 5-30 2015); booking for both venues is now open. www.wickedtour.co.uk