Sitting crosslegged in the window seat of a San Diego hotel suite, Katy Perry extends a bare foot – toes painted pink – across the ledge to show off a scar. She acquired the mark on her ankle in her teens, jumping off a roof at a pool party for a dare. “None of the girls would do it, they were all too busy sunbathing,” she shrugs, “and I would be the first!” She sits back, and adds coolly, “I like a nice dare from a boy. It’s fun. It makes me say, ‘Ha! You didn’t think I would do that.’ ”

Folks, meet Katy Perry – queen of “Ha! You didn’t think she would do that.” This is the Santa Barbara-born pop sensation whose risqué glam-disco anthem, I Kissed a Girl, has held the No 1 spot in America for a whopping seven weeks, and in the UK is currently outselling Kid Rock’s No 2 track by two to one, on downloads alone.

Praised by Madonna and adored by LA’s celebrity blogger Perez Hilton, Perry combines a saucy vintage look that gives Jessica Rabbit a run for her money with a tomboyish sense of rough and tumble that’s more like Minnie the Minx. Lindsay Lohan may have announced her lesbian relationship first, but it’s Perry’s flirtations with bisexuality that are making headlines – and not everyone is delighted.

In a recent tabloid scoop, Perry’s Christian parents describe her behaviour as “shameful and disgusting”. Given that they’ve spent their lives “sowing gospel seeds” as pastors around America, it’s perhaps fair to expect there would be a certain amount of dismay at a song which features the line, “us girls, we are so magical/ soft skin, red lips, so kissable”, accompanied by a video which has Perry, well, making the most of her own soft skin, red lips, etc.

But “praying” for their daughter every time they hear her single on the radio, as reported in the tabloids? Perry claims that’s nonsense. “My parents are definitely supportive and happy for my success,” she says. “I think if they did it their way, then maybe I wouldn’t be singing a song with this subject matter, but I’m an adult, I make my own decisions now.” Anyway, she says, mom and dad weren’t really that shocked: “It was more like, Katy’s always had something to say, now she’s saying . . . this.”

In any case, what is clear is that Perry feels that, bad girl persona or otherwise, she hasn’t turned her back on her upbringing. “I’m happy where I came from,” she says, in an uncharacteristically small, rather shy voice. “I love my family, and I like my upbringing, I think it was pure.” And then she laughs, rearranges her fringe and blinks her huge green eyes as if turning a page in her head.

It is, you suspect, as much steely determination as a naughty streak that’s brought Perry this far. Unlike the current crop of Disney starlets parading the charts (the dead-eyed likes of Miley Cyrus for example), Perry takes the driving seat in her career, making sure she gets approval on how she’s marketed. “I have people who are working with me who know exactly how I want to be portrayed, and my vision,” says Perry firmly. Which is what exactly? “You know,” she says, “ Me – Katy Perry – very sassy, cheeky, fun – cute, sexy and smart.”

Perry is, by turns, all these things. She is also very hard-working, currently playing on the Warped punk pop tour of stadium carlots; it’s hardly glamorous, but she’d signed on to it before what she describes as “light speed” success happened, and wanted to honour the commitment. Which means that besides the stream of interviews, photoshoots and radio sessions, she’s been playing live every day and holding signing sessions for hundreds of kids before the shows. It is exhausting, she says, “But you have to be really prepared for something like this, and have an almost superhuman sense about you.”

Ultimately, you’ve got to really want it. And Perry’s desire to perform is palpable. When the shoot moves from the hotel’s rooftop beach to its ludicrously titled “Extreme Wow” suite, Perry plays the part, clambering up on to window ledges, hot-panted bum in the air (“Take me like this!”), occasionally bursting into song. Her tendency to show off, she says later, is a result of middle-child syndrome: sandwiched between an older sister and a younger brother, she fought for attention by singing. “I was just getting whatever attention was left over,” she says, “and I loved people’s reactions, the looks on their faces.”

It worked a treat. By the age of 15, Perry’s singing in church had won her the admiration of a team of rock veterans in Nashville, who requested Katy be brought to Tennessee to polish up her songwriting skills. At 16, Perry released her first album, Katy Hudson (her real name, later changed to her mum’s maiden name for being too much like the actress Kate Hudson) – a Christian rock record.

But despite her relatively strict upbringing (no music, no MTV, no boys), life, inevitably, happened to Katy Perry. Her transformation started with fashion, inspired by Dominique Swain’s portrayal of Lolita (“I was like, I wanna wear little jumpers! And rompers! And look like a pin-up! Is that OK?”). And at 17, she left home for LA, where she began working with the producer Glen Ballard, the man behind Alanis Morissette’s Jagged Little Pill. That album was a huge influence on Perry. “She was totally a fly on every girl’s wall,” says Perry, “and that’s what I wanted to do, sing about honest things.”

Accordingly, Perry’s debut album proper, One of the Boys, deals in heartbreak, teen adventure, and puking into toilets. And then, of course, there’s UR So Gay, with its singular take on dissing a former boyfriend. “I hope you hang yourself with your H&M scarf,” trills Perry, before launching into its perky chorus: “You’re so gay and you don’t even like boys . . .”

Gay rights campaigners are, predictably, up in arms: “It’s official,” wrote MSNBC’s Tony Scalfani, “it’s cool to make fun of gay people again.” According to the gay rights campaigner Peter Tatchell, Perry’s lyrics “can be read as implicitly demeaning gay people. I am sure Katy would get a critical reception if she expressed comparable sentiments in a song called ‘UR so black, Jewish or disabled’. ”

Perry, unsurprisingly, objects to these accusations. “The fact of the matter is that we live in a very metrosexual world,” she says. “You know, a girl might walk into a bar, meet a boy, and discover he’s more manicured than she is. And they can’t figure it out. Is he wearing foundation anda bit of bronzer? But he’s buying me drinks at the same time!”

It’s meant to be funny, she says. “I’m not saying you’re so gay, you’re so lame. I’m saying, you’re so gay, but I don’t understand it because you don’t like boys!”

And Perry, of all people, says she welcomes parents wanting to censor their kids. “I think it’s important for parents to be in tune with what their kids are doing, from listening to this or that, to having a Grand Theft Auto weekend, where they’re digitally raping other digital characters.”

So how subversive does she think I Kissed a Girlis? “It’s just an innocent kiss!” says Perry, wide-eyed. “You know, girls are at a slumber party and they are hanging out with other girls, we’re all deathly afraid of that first kiss by that boy who we know is just gonna slobber all over our face.” And, she says, the song is about the “undeniable beauty” of women. “Some women are just so beautiful, that female or male, you would probably kiss them.”

Which begs the question, has she kissed a girl? “Oh yes,” she grins, “It was delightful.” When pushed Perry won’t say if she’s gay or straight, instead she describes herself as “metro-sexual”. The subtext being she’s young enough to experiment and old enough to know she’s straight but mainly is keen to keep all potential Perry-buying audiences interested by appealing to all persuasions.

And with that, she skips off to the bedroom in the suite, grabs the vacuum and starts singing Stop! In The Name Of Love to her rapper boyfriend, with whom she’s just exchanged “promise rings”. He’s lying on the enormous bed, next to Perry’s stylist and PA. We leave, as Perry jumps on to join them.

As the sun sets, Perry will clamber into a truck and join the Warped tour, parked in the massive parking lot of the Chula Vista Amphitheatre. Tanned punk kids crowd the site, flushed and drunk with the day; the sun glints on broken plastic cups and the sound of thrash metal wafts in the air. There, America’s No 1 pop star will front a real, punk-rockin’ band; jumping around and pulling Betty Boop poses to the delight of the kids – and Perry’s parents, who are in attendance, disgusted or otherwise. As her set closes, she’ll dive into the crowd, and the last we’ll see of Katy Perry is a pair of legs, splayed frog-like in the air.

The funny thing is, for all Perry’s hands-on muckiness, there’s something rather old-fashioned about her approach to performing. She’s beautiful, sure, but that’s not what makes her compelling – nor, really, is it the music. She’s more in line with Betty Page or Lucille Ball than the glossy likes of Jessica Simpson, or Lily Allen’s “keep-in’ it real” shtick. “I’m an old-fashioned entertainer,” she says. “You know, put on a show, don’t just sing!” Then she grins, and whispers it. “Put on a show!”

I Kissed a Girl is out now. One of the Boys is released on Sept 14, both on Virgin. Katy Perry plays Scala, London N1 (020-7833 2022), Sept 24 2008

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