Moments after Mira Grosin arrives, four of her schoolfriends requisition the table next to us, kicking back their chairs and studying the junk shop decor in an exaggerated display of inconspicuousness. "Oh my God, they're stalkers to me," she moans, throwing out her arm to block them from view. "They're true stalkers."

Grosin's portrayal of the pushy, mohican-sporting Klara is one of the joys of We Are the Best!, the new film by Swedish director Lukas Moodysson about three 13-year-old girls starting a punk band in 1980s Stockholm. But for Grosin, making the film already seems long ago. "I don't remember much because it was one-and-a-half years ago and I was only 11."

We're in Café String, an institution in Södermalm, Stockholm's southern island of sourdough cafes and hipster beards. "I live 100m around the corner, and my school is 50m away, so I'm pretty much 'here'. This is my place," she says. She was cast after Moodysson came to her school looking for the modern equivalents of his wife, Coco, whose comic strip about growing up in the area forms the basis of the film. It's easy to see why he chose her. Now 13, she arrives unaccompanied and greets me with a vigorous handshake.

"We have the same energy, me and the character Klara," she says. "We both talk too much and I can be a little bossy. It sounds bad, but she likes to own people, to be the leader." Before the film, Grosin knew nothing of punk. "I'd heard about it, but I'd never actually heard it, and I thought Elvis Presley lived in the 80s."

Nonetheless, she and the other girls worked with Moodysson to write the band's main song, Hate the Sport!, and improvised most of the dialogue. "We got to read the script just once before each scene, and he gave us a feeling we could work with, like 'you're going to be happy', and then we could improvise as much as we wanted. That's why it feels like it's me in the movie."

The film is a big-city counterpart to Moodysson's breakthrough, 1998's Show me Love, when he cast two untrained teenagers as girls growing up in small-town Sweden.

"It's easy to work with girls my age," Grosin says: "He kept telling us how easy we were to work with, because we are so relaxed to the camera."

Still, it wasn't all smooth. "We three girls were living in the same house and we were fighting a lot, like sisters," Grosin recalls. "It was hard to act happy and like I loved them when I was kind of mad at them both."

And far from hating sport like her character, Grosin actually wants to be a professional footballer as well as an actress, which is why she recently turned down an offer of a second film role.

"I thought it was too much. With school and playing football four times a week, I don't have time."

At this point, she notices a friend standing in the cafe window.

"That's my best friend, Liva," Grosin says. "Oh, she's going. OK, thank you very much, it was fun." And with a handshake, she's gone.

We Are the Best! opens in cinemas on Friday