With his third and final High School Musicalfilm about to open, Zac Efron feels ready to make the leap from teen heartthrob to serious actor. He talks to Michael Dwyer

WAITING FOR Zac Efron before our interview at the Dorchester hotel in London, it occurs to me that thousands - make that millions - of girls would give an arm and a leg (or two) to trade places with me. The night before, Efron brought Leicester Square to a halt, generating screams at Beatlemania-decibel levels when he and his co-stars walked the red carpet for the London premiere of High School Musical 3: Senior Year.

The movie is the first in that series to go on cinema release. The two earlier films were made for TV, building a huge international audience through repeat transmissions and on DVD, and spawning mega-hit soundtrack albums, videogames and books.

And the series has produced a hot new movie star in Efron, who appears entirely unaffected by such early success, perhaps because he has been acting since age 11 in California, where he made his debut in a community theatre production of the musical Gypsy.

"There was this wonderful woman who played Mama Rose in that and got a standing ovation every night, and she definitely was an influence on me in pursuing all of this," he says. "I didn't start acting professionally until I was around 15, when I began to get roles in TV series."

Efron played an autistic boy in the TV movie Miracle Run(2004), co-starring Aidan Quinn and Mary-Louise Parker. "They were the first movie stars I ever worked alongside. I was so excited." Two years later, Efron was cast as basketball-playing student Troy Bolton in the Disney Channel's first High School Musical, which became a cable TV phenomenon.

"I'm still amazed by how popular it's been, especially outside the US," he says. "I felt that was a testament to the themes kids were connecting to everywhere. It's very much a story of young love and the pressures and challenges of growing up, and I think that's a great backdrop for kids to be able to relate to. And then there's the music, which is universal."

The series presents an idealised view of life at East High School in Albuquerque, New York, where Troy and his classmates are attractive, talented, well-dressed and living in large, comfortable homes with caring, supportive parents.

High School Musical 3concludes on graduation day and marks the end of the series for Efron. "This has been very much a high school experience for all of us as a cast, as good friends. I feel that whatever else we pursued over the past three years, through music or movies, we've always had High School Musicalto come home to.

"The finality, now that it's over, is saddening. The last day on the set was very emotional. We knew it was the last time we would do it, but on that final day I couldn't believe it. It was a place where we'd loved to have stayed for the rest of our lives. But, you know, you've got to grow up someday."

In the film, Troy is faced with a tough decision between pursuing basketball or theatre at college, and being parted from his teen sweetheart Gabriella (Vanessa Hudgens), who's going away to university. The end of the series presented Efron with a similar dilemma, having to choose between singing and acting, or perhaps continuing to combine both.

Is it true that he turned down a recording contract offer from Simon Cowell? "That was so long ago that I can't remember all the details. It was more of a conversation we had along the lines of 'hey, let's make some music'. It wasn't formal. He was very nice and complemented my work in High School Musical. He asked me if I wanted to pursue music and I said no, not right now."

Efron exhibited his flair for song and dance again last year, playing sleek-coiffed Link Larkin in the effervescent 1960s-set movie musical Hairspray.

"That was a lot of fun," he says. "My character starts out quite shallow, but he gets cooler. And I got to work with Christopher Walken. We had a table reading, I sat across from him and I was just shocked to be sharing lines with that guy. The question I get asked the most is if I feel pigeon-holed by High School Musical, and the answer is no, I'm having fun doing a lot of things. I'm learning a lot from every film I do."

Efron impressively bridges the gap between teen heartthrob and serious actor - and gets to demonstrate his dramatic range - in Richard Linklater's Me and Orson Welles, which was warmly received at this year's Toronto festival and opens next year. He is thoroughly engaging in the non-singing, non-dancing central role of a starstruck 17-year-old who lands a minor part - the ukuleleplaying Lucius - in the young Orson Welles's adventurous 1937 Mercury Theatre modern dress treatment of Julius Caesar.

"I met with Rick Linklater," Efron says. "I had read the script. I thought it was a great opportunity because it was such a different role for me. It was exactly what I was looking for - a more serious and more challenging role. And I've always admired Rick's movies, especially Dazed and Confused."

Efron's character, Richard, is interestingly complex - seemingly very innocent but actually quite clever and ambitious. "He gets swept up in it all, but by the end, he really comes to admire Orson Welles, even though he sees all the fast ones that Welles pulls on everybody, and he recognises his intelligence and wit. And that's what Richard wants. He has his own little tricks up his sleeve."

Efron returns to song and dance in March when he stars in a screen musical based on the 1984 movie Footloose, which featured Kevin Bacon. And he has finished shooting 17 Again, in which Matthew Perry plays a 37-year-old who regrets all his mistakes and realises his dream of reverting to his late teens and starting over. Efron plays his 17-year-old reincarnation.

"That once again was a great change of pace," he says. "The toughest thing for me was to relate to playing a 37-year-old guy. That was the first character I've played that I just could not relate to in any way."

As it happens, Efron will be 21 tomorrow. "I'll still be on the High School Musicaltour," he sighs with a smile, "so I'll probably celebrate later."

Has his early fame restricted his freedom to go out in public? "Fan encounters continue to be nothing but pleasant," he says. "The downside, especially in Los Angeles, has to be the paparazzi. If one finds out where you live, they all know. If anyone did what they did without a camera, it would be considered stalking. And getting a picture is not enough for them. There has to be some kind of controversy around it. That's strange because I profess to being fairly boring outside of my job."

Really? "Oh, yeah." And does he have to cope with jealousy from other young men because he is so popular with girls? "Well, I've been there," he says. "I was in fifth or sixth grade when Titaniccame out and I'll never forget all the girls just losing it over Leonardo DiCaprio. I have to admit I was jealous.

"Then as I got older, I watched him progress and make great films, and he's one of my favourite actors today. He was able to make me change my opinion. I hope to be able to do that."

•  High School Musical 3: Senior Year opens next Wednesday