The Vue Leicester Square is lined by girls…and they’re screaming. Such is the excitement surrounding new young actor Alex Pettyfer and Stormbreaker, the novel adaptation following the adventures of teen spy Alex Rider, that it’s as if The Beatles have been condensed into a 16-year-old kid with a grade three haircut.

Inside the foyer however, the adulation in the cinema is reserved for author Anthony Horowitz. The boys clutching copies of the books probably couldn’t care less about the bloke kissing fans outside; for them, the pen truly is mightier than a semi-automatic.

“It’s nice that people haven’t forgotten that there was once a book and a writer and that’s how it all began,” laughs Horowitz wryly. “I’m used to being in a room writing on my own, so to be at a premiere surrounded by people and the media interest, I feel a bit like a fish out of water.” He shouldn't be: Miramax head honcho Harvey Weinstein has turned up with his family this evening so something's obviously gone right.

Finding the right boy out of 500 to bring his spy to life was an epic task, until director Geoffrey Sax came across Pettyfer: “He ticked all the boxes,” he says. “He looked great, he could act the part and he was very at ease with all the action sequences. We kept coming back to him, he kept on buzzing round our heads so eventually we just said ‘Let’s do it.’”

With six books already published and a huge waiting fanbase, stardom looks set for Pettyfer. “We kept saying to him, ‘Look, your life is going to change after the film opens’, but I don’t think anything can prepare you for what this is going to be like.” Mind you, the shrieking harpies outside the cinema could probably give him a nudge in the right direction.

Giving his suit an airing for the second week in a row after the Pirates 2 premiere, Bill Nighy is relishing playing another morally dubious part. “I have no feelings, I’m heartless and I’m the head of MI6,” he says of his character, Alan Blunt. “I don’t really consider it a problem whether he (Alex Rider) comes back in one piece, or several, or at all.” As with everyone here tonight, he’s full of praise for the young star: “Alex Pettyfer has done a wonderful job. I could never have done what he’s done at that age, I’d have run screaming.” That we’d pay to see.

Having already played one famous Horowitz in Clueless,  Alicia Silverstone relished the chance to get stuck into the English version’s story, as Alex’s guardian. No Brit japery onset for her though: it seems she's immune. “I have English parents so I’m quirked out,” she giggles. “But me, Bill and Sophie (Okonedo) had so much fun it was ridiculous. It’s not really a job, it’s more a vacation in the sun.”

The cheers outside have momentarily shifted to “Ewan! Ewan!” and Mr MacGregor himself soon waltzes in. As Ryder’s uncle in the film, did he – forgive us – play Yoda to the young star? “I sat him down as soon as he arrived on set and said ‘Now, listen here!’” he chuckles. “No, he’s got a very wise head on his shoulders has Alex, he didn’t need any advice from me.”

Having bounded around signing autographs, a newly-shorn Alex Pettyfer finally rushes in. “You get so much energy from the crowd and aargh! You really want to give it all!” he explains, beaming. Although, it seems even kissing fans can’t beat working with Ewan McGregor. “He’s fantastic, he’s an icon. Look at him, he gives such great performances even if he’s five minutes in a film.”

At 10 minutes in this one, McGregor gets to do just that, and is visibly enthused at starring in a UK action film. “I’m having the time of my life, it’s fantastic,” he says. “In Britain we’re not well-known for our action flicks, but there’s incredibly talented technicians, stunt crew, writers, directors and producers here so there’s no reason we shouldn’t be.”

IRA Zombies, your time has come.