Mary Kate Olsen - she wants to be alone

The Olsen twins are on different paths – and that's fine by Mary-Kate, hears kaleem aftab

Mary-Kate Olsen is 22 today. She can celebrate her birthday in the knowledge that she is finally succeeding in being seen as a separate entity to her twin sister Ashley.

The California-born star had, no doubt, planned that over the past year her public persona would stop being that of child star and marketing phenomenon. By playing a drug dealer in the third series of Weeds, and locking lips with Sir Ben Kingsley in Jonathan Levine's The Wackness, she was hoping audiences would no longer see her as one half of the Olsen duo; her fanbase would grow out of the pre-teen market; and she'd become as recognisable internationally as she is in the States. But that she has succeeded in all these departments has had little to do with her performances on screen.

It was, in fact, the death of the Australian actor Heath Ledger that ensured that she began making headlines. At first, it was erroneously reported that Ledger died in an apartment owned by the actress. Then it was revealed that Diana Wolozin, the masseuse who found Ledger's prostrate body, dialled Mary-Kate's number before calling 911. It was later reported that Olsen and Ledger had been casually dating for three months.

Ten minutes before I'm due to speak with Olsen, her publicist calls me, and politely makes it clear that unspecified personal questions will lead to the curtailment of the interview. We concentrate instead on Olsen's role as a stereotypical free-spirited hippie chick in The Wackness. Olsen starts off by talking about the project in a bland, matter-of-fact manner: "I read the script and really enjoyed it. I auditioned and I got the part. I go through the same process pretty much every time, and then I met with Jonathan. He is such a great guy and a very talented director. I had seen All the Boys Love Mandy Lane, I thought it was done really well and I was excited about working with him."

Even when I ask her about having to kiss the 64-year-old Kingsley, a man three times her age. Olsen is guarded: "It was sort of surreal. He is very professional and made me feel comfortable and it was pretty cool."

The trouble with talking about The Wackness is that Olsen is only in a couple of scenes. The first sees her dancing in a park, buying drugs, and in the second she kisses Kingsley in a bar phone-booth. But she says that playing Union was a good career move: "I thought it was the perfect film to be part of and she was fun to play with and exotic."

It's about time she had some fun. The phenomenal rise of the Olsen twins started when the pair were just nine months old and they were hired to play the single role of Michelle Tanner on the popular American television series Full House. When they finally left the show aged eight, the Olsen twins' popularity in the television arena was second only to Bill Cosby. But Mary-Kate is dismissive of the effect fame has had on her life: "I don't think any of it was about fame; working was my lifestyle and it was no different than going to school. It was just what I did."

The merchandising built around the pair was and remains phenomenal. Their images are seen on clothes, books, dolls, magazines and perfumes. When the girls were just six years old, the company Dualstar was set up to turn their brand-recognition into dollars. The pair were soon the youngest self-made millionaires in American history. After Full House ended, the girls' popularity continued to rise with straight-to-video films marketed as The Adventures of Mary-Kate and Ashley and the TV shows Two of a Kind and So Little Time. They also made one underrated cinema outing, New York Minute, and released songs and an album together. In 2004, when the pair turned 18, Mary-Kate and Ashley became chief executives of the company, and in 2007 Forbes ranked the girls as the 11th-richest women in entertainment.

When the Olsen twins decided to go to study at New York University, the event became a media circus. After her freshman year, Mary-Kate decided it was time to concentrate solely on acting. She says: "I took some acting classes in my freshman year at college and I found myself loving it again. I got a new agent and a new manager and started to read a lot of scripts and you immediately know what you want to be part of. It's something I hope to do well."

She clearly doesn't see herself as a bona-fide actress, even though she surprised many with her well-pitched turn as a drug-dealer in Weeds. "I don't think I'll ever have that extreme confidence," she reveals. "I do my thing and I'm a perfectionist and I don't usually watch what I do and I hope that it turns out well and people can appreciate my art and move on to the next thing. I watched the first two seasons of Weeds in two days or something and the third season I only watched the episodes I wasn't in, because I criticise myself and you know that, once you do it, it's done – why torture yourself?"

Regardless of her acting, the 5ft 1in star is extremely adept at deflecting questions that she doesn't want to answer, making the publicist's earlier call seem rather redundant. There has been much media speculation in recent years that the young star has battled with anorexia. When the topic is broached, she says: "I've never spoken about that. It's never been accepted or denied."

Olsen says that there is likely to be less collaboration with her sister in the future as, "I'm focusing on acting now and my sister is not." It is about time that the public began to see them as different people, says Olsen: "I mean, I've always thought of myself as a different person because I am, and I'll keep doing my thing and either people will catch on that we're two different people or they won't. It is important in my life to keep doing what makes me happy." For now, that means continuing to improve as an actor.

'The Wackness' opens on 29 August