The story of High School Musical is one of stupendously large figures. The two Disney movies have been seen by 400 million people since 2006, while 2.5 million children in the UK alone have bought the High School Musical books.

The soundtrack to the first film won a place in The Guinness Book of World Records as the only movie soundtrack to have nine concurrent singles in the Billboard Top 100. And now comes High School Musical 3: Senior Year, with more travails for the romantic leads Troy (Zac Efron) and Gabriella (Vanessa Hudgens), mostly at the hands of Sharpay (Ashley Tisdale), the biggest, baddest blonde of them all.

Except she isn’t big or bad. She is small and kitten-like in the half gloom of her suite at the Dorchester. Her skin is luminous, with little make-up, and she is dressed down — LA style — in beige sweat pants and a hoody. Everything about her suggests softness. Sharpay may stride into the cafeteria of East High in High School Musical 3 in miniskirt and pink-tasselled snakeskin boots bellowing her opening number I Want it All to the 150 dancing students, but Tisdale is as quiet and composed as a purring pussycat. Is she really like Sharpay at all? “I would hope not!” she giggles. “I love Sharpay. She’s the mean girl, but she’s hilarious and a lot of people like her — but I would hope that they wouldn’t say I’m like her.”

At 23, Tisdale is one of US magazine’s America’s Ten Most Powerful Girls in Hollywood, not only for acting (she picks up nearly £600,000 a movie) but also for singing (her album Headstrong got to No 5 in the Billboard chart) — and for running her own production company, Blondie Productions. So, when Tisdale describes why Sharpay is a role model for the next generation, she could also be describing herself. “Sharpay is really smart, she knows what she wants and goes out there and gets it. She’s very driven, and that’s a good quality. Actually, from an early age I have known what I wanted in life, too.”

Tisdale was spotted by a talent scout in her home-town mall in New Jersey at the age of 3, and had notched up 100 TV commercials by the time she was 8. “I always got the food commercials because I had the best ‘bite and reaction’, where you eat the food and are really excited by it.” Even before attending high school she had the leading role in a national tour of Les Misérables and had done an international tour of Annie.

She admits to being “not your typical 23-year-old. I don’t drink and I don’t smoke and I don’t go clubbing, I am mature for my age and professionally mature”. She has a boyfriend — Jay Murillo, a dancer in a boy band — and says: “I don’t look for guys in the business. I like people that do other things we can talk about. I have dated actors, but I like to have a normal relationship, which is hard because I am always travelling.”

I ask whether her early ascension into megastardom concerns her in any way, and whether the inevitable comparison with Britney Spears is worrying. “I think Britney is an amazing artist and I would never put her down,” she replies. “She’s amazing at what she does, but to be in the spotlight like that at 16 is hard. I mean I’m 23 and I’m dealing with it and it’s overwhelming.”

Tisdale went to a regular high school in Valencia, 30 minutes outside Hollywood, where she was the only working actress at the school. “People would see me on TV shows and would ask: ‘Is that you?’ And I would actually play it off like it wasn’t, because I really didn’t want to be ‘Oh that girl on TV who thinks she’s so cool’.”

Her drama teacher didn’t like her so she never auditioned for musicals, and admits: “I wasn’t popular. I was the new kid, I’d just moved from New Jersey to LA so I didn’t know anybody. I was very quiet and shy.”

Being in the Disney movies has provided a form of catharsis. “In High School Musical I got to be the popular girl in the school, which was really amazing.”

As the Disney press wagon prepares to roll out of London and back to New York for the film’s premiere, I ask her which is her favourite European city. “I would have to say London,” she answers tactfully. “It’s nice and cold, so you get to wear your winter jackets.” But it’s a sunny day, I point out. “Is it? I wouldn’t know, I haven’t been outside all day.”

High School Musical 3: Senior Year is on general release

Explore Film

• Film Reviews

• Trailers

• DVD

• London Film Festival

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010