Sunday afternoon in Toluca Lake, and I’m squished in a black SUV with six other people, having my ears pinned back by the most famous teenager in America. In the front passenger seat, maintaining a dainty Deep South elegance and coiffure in the Los Angeles heat, her grandma. In the back, two of her PRs. To her right, a handsome young chap in a vest and boy-band hair. To her left, balancing tape recorder on knee, me, the first British journalist to be allowed proper access to her world.

And in the middle, telling us all where to sit, fast-talkin’, gum-crackin’, God-fearin’ Miley Cyrus: star of Disney TV series Hannah Montana, top-selling pop singer, record-breaking live performer, record-breaking movie star, daughter of mulletted Nineties country singer Billy Ray Cyrus, and the titular head of a multi-platform branded corporation worth some $1bn. She recently received a seven-figure sum to write her life story. Miley Cyrus is 15.

We had gathered in the drive of the house Miley shares with her dad, mum Leticia, brother Braison, 12, and sister Noah Lindsey, 6. The Cyruses relocated from their home town of Nashville to this nice part of LA in 2005, after Miley landed the lead role in Hannah Montana. It’s a drama about a teenager who’s an ordinary schoolgirl named Miley Stewart by day but, “disguised” in a blonde wig, a big pop star named Hannah Montana by night. Miley Stewart’s dad is played by Miley Cyrus’s dad, Billy Ray, best known for his Achy Breaky Heart – after his country-music career faded, he became an actor.

Hannah Montana is a huge hit with tween and teenage girls. They love the escapism and idea of a secret (and supercool) alter ego. And Cyrus, a natural comic, is an engaging actor. For ordinary girls across America and the UK, she seems real and accessible, not some glossy and remote Beverly Hills type. Plus, the pop songs she sings in her husky, mature voice are good, too.

In real life you can see the appeal of Miley Cyrus. She talks fast, is unselfconscious, a genuine teenager with genuine teenage appeal that is impossible to manufacture. Her charm is in her unguarded candour. A teenager who speaks her mind will speak to her audience.

Originally, the schoolgirl character was called Chloe Stewart. But the Disney bosses who put adolescent Miley Cyrus through a gruelling audition process that lasted nearly two years ultimately decided to name the character after their new hireling. Why?

“They just thought I was like the character,” shoots back Miley, then instantly reverses the comparison. “The character was so much like me – because she’s really wild and fun and doesn’t really give a care. And that’s very much how I am. What you see is what you get. I’m not really worried about stuff, I just like to get stuff done. Same thing: she’s really, really ambitious and wants to just go out and change the world. Hannah Montana doesn’t just wanna be a pop singer. She doesn’t just wanna be something that’s hot right now. She wants to go on for ever. Same thing for me.” Of course, all teenagers think they’re immortal. The difference with Miley Cyrus is that for her, going on for ever means being a star for ever.

This evening Miley is headlining a concert at the Universal Amphitheatre, part of the film studio’s theme-park complex. The sold-out event for 6,200 screaming girls (and some earplug-wearing parents) is in aid of City of Hope, a California hospital specialising in cancer treatments. The bill comprises three acts-cum-brands fundamental to the current pop-cultural supremacy of the Walt Disney Company: Demi Lovato (star of the recent Camp Rock movie, now making her first album), the Jonas Brothers (also Camp Rock-ers, but a stadium-filling boy band, too), and Miley Cyrus.

In the driveway of the Cyruses’ low, ranch-style house, the SUV doors slam, the engine guns, the gates hum open. We embark on the ten-minute drive to Universal Amphitheatre. Miley starts talking. Fast. Very fast. This, I will discover over the ensuing five hours in her orbit, is how she always speaks, whether to reporters or her mum or Disney top brass or the sick children who visit her dressing room post-show. I soon develop a form of Miley lag. It’s like jet lag, but it makes your head spin more. No wonder this child-star-entrepreneur gets so much done. As she speaks, sentences fall over half-sentences. No thought goes unuttered, it seems.

“I just think it’s really important,” Miley says of her support for City of Hope as we exit her family property, a fat envelope from a fan wedged in the gate’s iron bars. “Just because so many kids have all these dreams, and I know if I was little – I started when I was six or seven years old… The people that get diagnosed with these type of cancers and stuff at such a young age, they never feel like they have an opportunity to really dream – THAT WAS THE GUY I WAS TELLING YOU ABOUT, THAT STALKED ME!”

Feisty Miley, a short kid with a woman’s flowing locks and a curiously throaty voice and still-chewy Southern accent, is now shouting and craning round in her seat, pointing at a shifty-looking bloke standing on the corner just down from her house. Her PRs look back too.

“THAT GUY, SERIOUSLY, NEEDS TO GO TO JAIL. UM, OK. HE STALKED ME, I’LL TELL YOU ALL ABOUT IT.

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