Mischa Barton, snapped last week in leather leggings at London fashion week in close proximity to Josh Hartnett, Ben Stiller and the Geldof girls, seems yet more blonde fodder for the tabloids.

Beloved by a generation of teenage girls – thanks to her role in the long-running teen soap The OC, in which she played the doomed belle Marissa Cooper – she has the same air of fashionable detachment as her on-screen incarnation. Cooper was a Californian babe who didn’t do much more than “hang” in Ugg boots and shop at Chanel (until she unfortunately died).

They also talk in a similar way; the only obvious difference is that, off screen, Barton chooses not to hang in California herself. “It’s harder for me to find people in LA who are worth hanging with and who are hangable,” she says.

Instead she prefers to hang in Paris, where she has a flat in the chic 4th arrondissement. “With all the privacy laws there I haven’t seen paparazzi since I’ve been there.

That’s why I’m there, oh yeah. Here in London my friends are so taken aback by how things are.”

She’s careful not to be too dismissive of the attention, however: “Some nights it feels that everyone wants a piece of you – you want a drink with a friend and someone is going to be in your face with a camera – but the upside is, hey, at least they’re your fans, they appreciate you; and the more people like you, the better it is.

“The OC has been off since early 2007,” she continues, “and three times last night I was getting, ‘Oh, when Marissa died, I cried’ – so I’m, like: cool! People still care.”

With her grasp of teen talk, her long, honeyed locks and heartshaped face, it’s not surprising that the beautiful, British-born Barton, 22, was chosen to star in a show that dramatised teen life in Orange County, California. Faced with a rare patch without an acting job, she has wasted no time in using the fallow period to market her own fashion label.

“I’m not filming until November, but with economics the way they are in America, there aren’t that many films getting made,” she says. “There is definitely a huge downturn and a sour feeling in LA. So I’ve been spending all my down time making handbags.”

The bags (which, unlike other celeb-branded gear, mercifully retail from only about £20) arrived in London department stores this summer. Barton has determined they will be sold only in the UK and Japan – “I like to be in control.”

Today she’s just come back from South Africa, where she visited a school in Soweto as part of her role as ambassador for Save the Children. She describes the trip as “life-changing” and her work for the charity as a “calling”.

“I went to see this little boy, Tembinkosi,” she tells me. “He was so sweet and shy. He was only eight . . . He has no toys. He sleeps on the floor in a room full of dirt and he only gets one meal a day. It was terrible. But if I can raise money, that will make a difference.”

So another photogenic star uses vulnerable children in Africa for a profile boost between film projects? “Well, if cynical journalists think that, I think that’s pretty jaded and sad,” Barton retorts. “If someone takes the time to actually go, I don’t think people sitting on their bums writing articles have the right to say anything, actually.”

And not just journalists. “I see so many celebrities who don’t take the time to do anything, which is pretty terrible in its own way,” she comments. “What’s the point of celebrity and all this stupid press we get? I think young people should be more active, vote, be more involved in charities.”

Recently, she says, she was involved in a campaign to knit hats for babies and roped in her friends to get knitting alongside her. Her on-screen persona may sum up the dumbest of consumer-society youth, but Barton doesn’t seem to have time for it herself. “As a 22-year-old, I think the society we live in is pretty sad. And I’m not very proud of what’s happened to my generation,” she says.

“I was at this party and someone wrote up on the wall, ‘Nothing relevant has happened in my generation.’ Well, that’s not true – it’s just the things that have happened, such as Iraq, you aren’t very proud of. I think my generation is obsessed with things you can get quickly, a short attention span, a blog about anything, fame for 15 minutes.”

She knows celebrity can be useful, though. “I don’t care what people think about me,” she says, swinging that beautiful hair. “I just want to draw attention to the issue.” She has personally funded a $2,000 (£1,100) irrigation system for Tembinkosi’s school so it can create a vegetable patch for the 800 children it serves.

“Once you make money,” she says, “you have a responsibility to people, and for someone like me – whose career has become very public and whom everybody is photographing and gossiping about anyway – it’s nice to have something which you can draw attention to and for which you can use your fame.”

For more information on Save the Children go to www.savethechildren.org.uk

Explore Celebrity

• People

Times recommends

• Live fashion advice: how to dress for party season
• The A-Z of winter style rules
• Could safe havens prevent teenage stabbings?