Are you a superhero?” Time stands still and my pulse quickens as I read this in the paper. Magnus Scheving, the creator of LazyTown, and some big-shot producers are searching for a Sportacus to star in their nationwide tour of LazyTown Live! Sportacus is the super-fit superhero from the hit TV show and icon to children all over the world. I just have to go to an open audition in London and do a short aerobic/gymnastic dance routine and sing the top-selling Bing Bang Song. I already know the tune; it’s been messing with my head for some time.

I’ve had my calling and I’m stepping up to the plate. All right, I’m at the top end of the age range at 42 and the knees are prone to inflammation after six or so stairs, but I spot a big chance. As I walk through the stage door of the Criterion Theatre, I feel a surge of nerves. I nearly bottle it, but feel better once I don my Sportacus kit (Argos, �12.99) and chant his mantra: “Think, move, go.”

The panel of producers, directors and Scheving, who plays Sportacus on television, are dotted randomly around the stalls. They are kind and generous, not at all like the TV talent show bullies. A camp young fellow minces to the front, but he’s more lay-about than LazyTown. According to a veteran of West End musical auditions that I consulted earlier, he makes a common mistake: he doesn’t have a clue about the show for which he’s auditioning.

LazyTown is shown in 109 countries and the live show will play in more than 50. Scheving, who must be worth more to the GDP of his native Iceland than fish, looks like a Scandinavian man trapped inside an Aussie lifeguard’s body. The show is on CBeebies and Nick Jr in the UK, and appears to be bland Americana at first glance. But look again and it hooks you in, along with your child. The extraordinary pace and energy are created with a tight script and a clever mix of live action, puppetry, CGI animation and an insistent soundtrack. Each 25-minute episode costs �500,000, making it probably the most expensive children’s television in the world.

But what really makes it a hit is good old-fashioned character and story. This disguises the whopping great message of healthy eating and exercise. There is a pink-clad, outsiderish eight-year-old girl called Stephanie, who along with the other kids in LazyTown(the puppets) is easily distracted into bad ways by Robbie Rotten, the villain. Like all good stories for children, he represents the dark, sometimes philosophical and humorous side. Then there is Sportacus, an accessible, slightly flawed, even comic, hero. He is capable of amazing physical feats, positive thinking and the ability to turn around any situation, all down to exercise and “sportscandy” (fruit and vegetables).

Scheving turns and spots me lurking in the shadows at the back of the stalls. He moves towards me with a series of SAS-style gymnastic leaps over the seats. He lands beside me with graceful stealth. “Hi, I’m Magnus.” Unlike many self-made men, he’s good at self-deprecating wit and reveals a grounded side. He tells me that he built the house that he shares with his wife and three kids from scratch. Bloody hell, andhe gets my little girl to eat broccoli.

I ask him how on earth did a carpenter and the 1994 European aerobics champion get to be a worldwide improver of infant health with a complex, and it has to be said, bizarre show? Did he plan it all to turn out like this? “No and, in a strange way, yes.” And here he becomes intense. “One thing led to another but I knew that I had to do something to fight childhood obesity.” The hit show didn’t arrive fully formed. It was first a book, then a musical. What did he think of Jamie Oliver? “We are going to meet in August, but he has a different approach, facing older children head-on – I think that when a child is 8 it is probably too late.”

All of a sudden, I’m called to the stage and once again the nerves kick in. I look down and see the eight or nine faces spread about; as I’m one of the last, they all look a bit glassy-eyed. The smiley one who looks like a chubby schoolboy adopts a fixed grin. I feel bare, naked, raw. Imagine sitting in a normal interview for an office job then just standing up on a whim to sing the Bing Bang Song.

Thankfully, the director opts for some work with the script. Magnus tells me to pull my Sportacus hat down a bit. I’m so taken with him, I try way too hard to impress, so when I have to pretend to motivate a theatre full of kids I sound as if I’m drunk. “Try again but go slower and with an Icelandic accent.” Now I sound like a Dutch porn star. Next thing, Scheving is flying through the air-landing next to me to put me through my paces: jumps, twists and an aborted one-arm press-up. This is better until the inflamed knees and panting kick in. I see that next to his precise and contained panther, I’m a sloppy old arthritic labrador.

“Thank you, we’ll let you know,” says the director. They really do say that, and it’s all over very quickly. As I’m leaving, the photographer is snapping away at Scheving leaping over more seats, and then I see exactly how Sportacus came into the world. A member of the theatre staff intervenes: “Excuse me mate, stop jumping over the seats.” Suddenly I see Sportacus imploring Stephanie in his Icelandic tone: “No, you see, all you have to do is this, not to touch any part of the seat.” His next jump is magnificent and I think for one horrible moment he is going to teach a 16-stone man with a spanner how to jump seats. No, he’s not having it, and he throws us all out. Our superhero has met his match, we’ve found Sportacus’s kryptonite: the lardy British health and safety jobsworth. For once, Robbie Rotten wins.

LazyTown Live!, Hackney Empire, London E8 (www.lazytownlive.co.uk 020-8985 2424), Oct 4-6 and then touring nationwide