It can be hard work being a Hollywood hero. Just ask Ben Barnes, a.k.a. Prince Caspian, star of the billion-dollar-grossing Chronicles of Narnia franchise.

There's the grinding tour of world premieres ("Travelling for three months, sitting on your own in a hotel room in Taiwan, thinking, 'what am I doing here?' Just weird.") and the red-carpet appearances where your every move is greeted by an electric storm of flashbulbs ("I don't think there's any way of ever being comfortable with it. You put on a perma-grin, which is so unnatural, and the lights make you squint. I get very anxious."). There's the frankly disconcerting experience of seeing your very own toy action figurine for the first time. "My own was absolutely awful, shamefully bad. I was a bit upset by that. How many times do you get an action figure made of you? Mine looked like a cross between Adrien Brody and Javier Bardem – but ill." There's the tireless and tiresome conveyor belt of press junkets and interviews.

"They take as much out of you as filming does", sniffs Barnes. "I thought, 'it's only chatting, how hard can it be?'" As it turned out, the New York round of Prince Caspian promotion very nearly wiped this swashbuckling regent out: he did 90 interviews in one day, woke up the next with no voice and found he had to do 70 more, dosed up on a cocktail of vitamin C, honey and lemon, and Chinese potions. "I didn't really have a choice. The film was called Prince Caspian, there was no hiding." Appearing on Jay Leno's sofa was a rare highlight. "Have you seen it? It's definitely the best interview I've done..."

And then there are the fans, the prolifically letter-writing, largely female fans. Barnes spends "thousands of pounds" replying to the sacks of mail he receives each month. When he casually thanked a fan in an interview for throwing a pair of "Hello Kitty" boxer shorts at him as he arrived at Tokyo airport ("It was a bit creepy, but they were clean and I'd been travelling for three months..."), his mailbox was swiftly inundated with cutesy cartoon-cat underwear. "What part of a 27-year-old English man wants that?" he asks, incredulous. "I don't know what to do with them, they just sit there in a pile."

Not that Barnes is complaining, of course. It's just that all this fanfare and adulation has happened rather suddenly and the slight actor from Wimbledon hasn't quite caught up. "Somebody needs to give me an alternative to the word 'overwhelming'. People don't want to hear that. They want to hear that you enjoy it." Who wouldn't be overwhelmed, though, at the sight of themselves brandishing a sword on a 10-storey billboard on Sunset Boulevard? Or at being the centre of attention at the UK's largest ever film premiere, which drew 10,000 Narnia fans to the 02? Or at a head-spinning $419m in box-office receipts?

One year ago, few had heard of Barnes. He had a fleeting appearance in the Hollywood fantasy film Stardust and a West End play under his belt. Prince Caspian's casting director was in the audience for The History Boys one night and saw something in Barnes's swagger as the peacock of the class, Dakin. They called him in for an audition and a week later he had the part. He was immediately dispatched to "Narnia boot camp", where he learned to gallop, sword-fight and bellow war-cries, before a seven-month shoot in New Zealand and Prague.

Since then he has snapped up two more leading roles. First as the foppish foil to Colin Firth in Easy Virtue, Stephan Elliott's brash reworking of Noël Coward's play, and, secondly, he has just finished filming the lead role in Oliver Parker's upcoming version of The Picture of Dorian Gray with Firth (now a "close friend") and Rebecca Hall.

As if further proof that Barnes has "arrived" were needed, in March he took part in one of Vanity Fair's coveted photo-shoots, recreating West Side Story with Jennifer Lopez and a cast of hot young actors including Twilight's Robert Pattinson and High School Musical's Ashley Tisdale. It was just as fun as it looked, he says, proudly showing me a dancing scene, now the screensaver on his iPhone. "I didn't really know what was going on", he admits. "I felt like the odd one out. I think I always will a bit."

Today, arriving at his agent's office and blaming his late arrival on the Piccadilly line, Barnes doesn't really look like a star. Tall and slim and dressed in a white T-shirt, leather jacket, jeans and biker boots, he's sporting some patchy stubble and "ridiculous" long hair in preparation for the Prince Caspian: Voyage of the Dawn Treader shoot in July ("I do not want extensions again."). The only sign that Hollywood has got to him is a set of dazzlingly white, perfectly straight teeth which he flashes frequently in a smile that has charmed even the bitchy gossip-hound Perez Hilton, who frequently posts red carpet pictures of the "dreamy" actor on his showbiz blog. Not that Hilton has many occasions to do so: though a frequent flier to LA, Barnes still lives in London with his brother and prefers a night at the cinema or a gig over what he calls the "gossip-mag lifestyle". He is, though, finally coming round to the idea that he might have made it. "I've always felt there's an excuse for me getting the job, that it can't actually be my acting. With Narnia, they were running out of time and needed someone who was good with accents: I can do accents. It's happenstance: I haven't got this of my own right", he shrugs. "And then I got Easy Virtue and The Picture of Dorian Gray and couldn't come up with an excuse for them."

Barnes was born in leafy south-west London to a psychotherapist mother and a professor of psychiatry father. At King's College School, his precocious classmates included the actor Khalid Abdalla (star of The Kite Runner) and if.comedy Best Newcomer, Tom Basden. Barnes played drums in the school jazz orchestra and put on Motown nights with Lizzie (sister of Rob) Pattinson from Wimbledon High, a girls' school down the road. His first taste of performing came when he joined the National Youth Music Theatre (NYMT) and appeared, aged 16, in the West End in Bugsy Malone alongside other rising stars Jamie Bell (Billy Elliot) and Sheridan Smith (Gavin and Stacey).

"When I finished school, everyone wanted to go to a good university and become a lawyer or a doctor. My A-levels were sort of chosen for me. The only thing I'd ever chosen for myself was the NYMT. I needed some time to think." Though his parents wanted him to go to university, Barnes took two years out, trying out singing and television presenting, and cooking up a project with the entertainment svengali Simon Fuller to open a jazz club and launch an album.

When that came to nothing, he enrolled at Kingston University to read English literature and drama – "but the only thing I wanted to do was direct plays, be in plays and take them up to Edinburgh". In what one imagines was a moment of student inebriation, he briefly joined the boyband Hyrise, who were longlisted as the UK's Eurovision entry in 2004. "We had one song and we sang it once. It was a stupid side-project for, like, a week", he sighs. "It's the worst thing I've ever done but I didn't expect anyone to ever see it." Their one performance is immortalised on YouTube.

Stardust was his break into the major league and led to Bigga Than Ben, a low-budget independent British film about two "pieces of Russian scum" trying to make a living in London, milking the system with illegal jobs, drug dealing and rent dodging. "It was shot for £200,000 over four weeks. We got the bus everywhere and got changed in phone boxes", says Barnes proudly. "The director saw Stardust on my CV and said: 'So you've just worked with De Niro?' I said yes. Didn't meet him once of course..."

It has not always been an easy ride; Barnes already has one dispute under his belt, when he left The History Boys six months into its West End run to play Caspian, annoying the National Theatre. "I'm still hugely regretful about it and I still haven't heard a peep out of them. I didn't feel like I was leaving anyone in the lurch but I did feel like I was letting people down. And I felt like I was going to vomit for three and a half weeks between getting Prince Caspian and leaving. It was always my dream to work at the National: I'd been going there with my Dad since I was 10. I'd love to round it off one day by going back. It's still the most gratifying job I've ever done."

Prince Caspian, he implies, was not quite so gratifying. "It's more to do with the contrast between the marketing and what I was asked to do. I was trying to do this awkward character who wasn't sure he wanted the responsibility of being a leader. I don't think that tested so well, so they tried to tweak him into a hero." Did they Hollywood-ise him? "Just a little. Yeah. Which is completely fair and it made a lot of money so they know what they're doing. But at the same time, as you can see by all my other choices, it's not particularly what I'm going for. I want to play characters that are interesting to watch."

Still, the blockbuster has opened doors. "People want to meet me more. It's something in people's eyes when you go into a meeting" – and Barnes is now firmly part of a powerful Brit pack of young actors cutting a dash through Hollywood, alongside Pattinson, Rupert Friend, Jim Sturgess, Dominic Cooper (who also made his name playing Dakin) and Andrew Garfield. He was recently rumoured to be lobbying hard for a lead in the massive Twilight movies. "I've never been precious about stuff. I'll ring up directors and ask them very forward questions. I don't really like the politics of it, I'd rather get involved." Next up, he's filming the Narnia sequel, and a Memento-style thriller in Boston and a British film are in the pipeline. Meanwhile, he dreams of making a film of Jesus Christ Superstar with the theatrical American Idol runner-up Adam Lambert. Is he ambitious? "I don't feel like I've got on the second rung yet, that I've got going. I don't really know who I am as an actor: the best thing would be to experiment with it for the next 30 years and never really find out."