We had a sneak peek backstage at the iconic musical (Picture: Emma Cattell)

Out of all the spectacular musicals currently in the West End, there is one that causes its audience to gasp in astonishment like no other.

This jaw-dropping moment arrives at the end of Act One with Elsa’s character-defining anthem Let It Go, the hottest song in Frozen.

As the number builds to its rousing climax, Samantha Barks’ Elsa draws courage from its life-affirming message – be true to yourself, let go of your inhibitions and, um, release your inner superpower.

It is a song delivered by the star as a force of nature – a cyclone turning into a hurricane. Then it happens. As Let It Go reaches its crescendo Elsa physically, instantly and stunningly changes into the fully-fledged Ice Queen she was born to be. But don’t blink. The transformation is instant.

Samantha Barks belts out the tunes as Elsa (Picture: Johan Persson)

Elsa’s iconic gown before it gets its big on-stage moment (Picture: Emma Cattell)

‘When I do Let It Go it’s a real team effort,’ says Samantha as the star checks her make-up and her character’s long blonde hair in the dressing room mirror. ‘There is a lot of magic that happens in the show. More than any other I’ve done before,’ she adds.

As we talk, the award-winning musical is already underway. A speaker is piping the show’s progress into Samantha’s room.

Despite the rollicking speed of writer Jennifer Lee’s storytelling we can hear we are still in the early scenes in which Elsa and Anna are children. Meanwhile, Samantha is the embodiment of professional calm as she adjusts THAT dress.

The gown must be among the world’s most merchandised items with countless Elsas owned by as many children. The dolls out-sold Barbies at one point. But this dress is the real deal.

Samantha loves looking out at the audience dressed as little Elsas and Annas (Picture: Emma Cattell)

The costume (the production has three of them) glistens like an azure sea under a hot sun. There are 954 sew-on stones, 10,800 crystal beads and over 6,000 Lochrosen and Hotfix crystals all of which take 41 days for one person to bead.

The dress is first seen in the show when Let It Go reaches its climax, a reveal so secretly guarded a Disney executive jokes I’ll have to be killed if I’m told how it is done.

This is the part of the show where Michael Grandage’s production narrows its focus on Arendelle’s newly crowned queen and no one else. It is the moment everyone has been waiting for.

How does it feel to perform one of the best-loved songs in musical theatre? ‘You feel like you’re piloting the ship,’ says Samantha. ‘There’s so much that goes into the show but in that moment you get the chance to be the captain. What’s special is looking out at all these faces all dressed as little Elsas and Annas.’

For those waking from a decade-long coma, Anna is Elsa’s sister who is played by Stephanie McKeon to romcom perfection.

Welcome to the dressing room (Picture: Emma Cattell)

Stephanie McKeon plays the loveable Anna (Picture: Emma Cattell)

I am now being led to her dressing room, through the labyrinthine corridors of Drury Lane, one of London’s grandest and I’m quickly discovering, largest theatres.

Samantha’s is like a suite. There are sofas and an armchair but her stage cue is not too far off now so this is no time to kick back. Stephanie does her vocal and physical warm-ups, and sits at the mirror, relaxed but ready.

Earlier that day I had watched Samantha and Stephanie rehearse a special performance for the Olivier Awards and the two stars seemed as close when they were out of character as they were as Elsa and Anna.

‘I don’t think we could fake that,’ says Stephanie. ‘From the moment we met we really did hit it off. Samantha was cast before me and texted as soon as I was given my role.

‘We went for a cup of coffee, that turned into prosecco and then dinner. Since then we have genuinely been like sisters and spend time together outside the show. I think that’s important because lots of people say they can feel our chemistry on stage.’

‘We are just so close,’ Samantha had told me earlier. ‘Even our dogs love each other.’

Ashley Birchall has an impressive costume as Sven the reindeer (Picture: Emma Cattell)

Being Sven is a bit more difficult than playing your average pantomime animal (Picture: Emma Cattell)

Elsewhere in the theatre two of the non-human stars of the show are also getting ready. Somewhere in the basement bowels of the building Ashley Birchall is limbering up for the most physically demanding performance of the production.

His limbs are extended with stilts as he climbs into the body of Sven the reindeer, companion of and cargo carrier for Kristoff the ice seller, not a job much in demand after Elsa froze Arendelle.

Few characters get a warmer reception from the audience than Sven when he first appears. Except perhaps Olaf, the guileless, eternally loyal friend made of snow who Elsa conjured from her powers when she and Anna were children.

‘My neck has got wider,’ says Ashley, as a strap attached to a bungee is attached to his forehead so he can bear the considerable weight of Sven’s shaggy head.

At the other end of both man and beast Ashley’s feet have already been bound into a balletic point position. This creates the dogleg shape of Sven’s hindquarters. Meanwhile, Ashley’s forearm extensions allow him to control the reindeer’s expressions by twitching ears or fluttering eyelashes.

‘Eighty per cent of the weight is on my arms,’ says Ashley, before Sven’s head is finally put in place. Even now the animal seems just too big for one performer to inhabit.

But then Sven is a very different species from the traditional two-person pantomime horse, just as Olaf is a world away from, well, anything.

The snowman is brought to life by Craig Gallivan who is both Olaf’s puppeteer and his alter ego. The relationship between the two is strangely symbiotic, Craig agrees.

Getting into costume (Picture: Emma Cattell)

It’s all in the details (Picture: Emma Cattell)

‘The audience are sometimes watching me, sometime watching the puppet. They almost forget I’m there. But there are also moments where I come out of character and can respond to something Olaf is doing by commenting with my own expression. Then I go back in character and I am Olaf again. It really is weird dancing this line.’

By now the grown-up Elsa and Anna are on stage and we are building to that wow moment.

I am backstage now with Tim, head of wardrobe, who tells me he knows someone who came to the show especially to see Elsa’s famous transformation.

But at the crucial moment she thought she heard her name mentioned by someone behind her and in the nanosecond she averted her gaze from the stage, Elsa’s moment had come and gone.

‘Tell your readers,’ says Tim, ‘be warned!’

The ‘wiggie’s

In the wardrobe village where most of the costume changes take place, nine dressers are helping the cast change in and out of the show’s 154 costumes.

There are a lot of Elsa wigs backstage (Picture: Emma Cattell)

And a dedicated team to look after them all (Picture: Emma Cattell)

Meanwhile 64 wigs are prepared by the show’s ‘Wiggies’.

Some of them move through the backstage gloom with the efficiency of a pit stop crew to execute the fastest of hairdos – just 18 seconds to swap one performer’s wig with another.

Crossing the ice bridge

‘The end of Act One is the busiest across all departments,’ explains Assistant Stage Manager Sally Inch in a hushed voice.

The ice bridge is an impressive bit of set (Picture: Johan Persson)

And here’s how it looks backstage (Picture: Emma Cattell)

Watch out – it’s slippery! (Picture: Emma Cattell)

It was Sally’s job to figure out how cast, crew and props can move safely around and behind the stage. This includes the massive ice bridge, which is wider than the performance space when fully extended.

It is thanks to Sally that everyone knows exactly where to be and when, including
me, who she keeps clear of the traffic.

The ice strike

Being behind the scenes during a performance of a big West End show is like being aboard a warship at battle stations.

Every move backstage is as choreographed as every move on stage. But this is particularly true of Frozen, which has a piece of bespoke kit that looks like a doomsday weapon designed for a future war.

Incredible ice structures make a big impact on stage (Picture: Johan Persson)

There are all sorts of technical effects to coordinate (Picture: Emma Cattell)

Black and as tall as a bus it takes four backstage crew to move the dangerous machine into position, and another three to operate it. When fired it produces what the crew call the Ice Strike.

It happens in the ballroom scene where the just-crowned Queen Elsa loses her temper and her hitherto suppressed powers conjure giant shards of ice that plunge into the palace floor. The audience have no sense of what is about to happen. But backstage the tension is palpable as every department in the production continues their work.

‘There is pandemonium after the Ice Strike because the citizens of Arendelle run off [stage] in different directions,’ says assistant stag manager Sally. ‘Keep behind the hazard line,’ she adds, as the Ice Strike machine is moved into position like a medieval catapult.

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It stops just out of the audience’s view. Behind it is a container of compressed gas, which will propel the shards onto the stage. A member of the automation team keeps his hand firmly on the machine’s Dead Man’s Handle.

If for any reason he sees someone out of position and in the line of fire it is his job to let go of the handle and abort the scene. But all is clear.

At one stage, in a flash of anger, Elsa loses control of her powers. Backstage Head of Automation gives the command to fire and with the apparent speed of a lightning bolt shards of perspex ice shatter the peace of Arendelle.

The kingdom’s citizens gasp in awe, as do the audience.

Book tickets from the Frozen the Musical website.

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