The Performance: Selena Gomez

Thursday, Selena Gomez turns 18, officially entering into adulthood. But instead of partying, she’ll be busy working, promoting her new film “Ramona and Beezus,” which opens Friday and provides her first real movie role.

Truthfully, Gomez has already been acting like a grown-up for a while. Four years ago, she began filming “Wizards of Waverly Place,” the hit Disney Channel show about a girl with magical powers that has propelled her to the top ranks of teen stars. Today, like other young Disney powerhouses, she has become a bankable brand: She has a clothing line, is about to release her second album and is an ambassador to UNICEF.

Now comes her new film, based on Beverly Cleary’s beloved children’s books, in which she plays the iconic literary character Beezus Quimby. She chose the project because she felt it was an appropriate family-friendly vehicle that would be accessible to her fan base.

“I wanted something that was just perfect for my audience. I didn’t want to do anything that was too much off the rail just yet,” said Gomez last week, sitting in a trailer on the CBS lot, where she had just filmed a singing appearance on “ America’s Got Talent.”

It was outside a trailer like this that Gomez greeted the group of admirers who would come looking for her daily on the Vancouver, Canada, set of “Ramona and Beezus,” said the film’s director, Elizabeth Allen.

“There would be hundreds of kids waiting outside set every day with these wide eyes I hadn’t really seen before,” said Allen, who acknowledged that during the casting process, she wasn’t really aware of Gomez’s star power. The actress first auditioned for the role when she was 15, when she wasn’t “quite as popular,” said the filmmaker. But the movie took a long time to get greenlighted, explained Allen, and by the time she was ready to cast Gomez, the teen had her pick of other films to work on.

“And she chose this, which was not a big payday, and she’s a supporting role,” the director said. “I think many other kids in her position would have taken the money and the leading role, but she felt there was a pedigree to the property, and she had never been in a feature film. She wanted to work with a high-caliber cast [including John Corbett, Ginnifer Goodwin, Josh Duhamel and Bridget Moynahan], and soak up all their abilities.”

Explaining why she decided to play the smaller part of Beezus — a studious teen who is continually embarrassed by her pesty sibling, Ramona — Gomez sounded startlingly practical.

“Any role at this point I was really grateful to have, and this is my first theatrical role, so I wanted to make sure I really learned and was on my best behavior,” she said, dutifully autographing a stack of her own glossy headshots that would be given to fans. “Not that I’m not always.”

Gomez, it’s clear, is very much a product of the well-oiled Disney machine. She rarely seems to go off-book, smiling while she constantly reinforces how appreciative she is for all she’s been given and for her fans’ support. She would like to model her film career after that of Rachel McAdams, who is known for choosing roles in high-profile projects and manages to stay out of the tabloid spotlight.

Her favorite pastime, she said, is watching reruns of the television sitcom “Friends.” And she feels “hopeless” without her mother, who had Selena when she was 16 and is now her manager.

And yet, she doesn’t see herself as the good girl of the Miley Cyrus school.

“No, I never wanted that title. I don’t want that title. I don’t want any title,” she said, twirling an oversized ring around her finger. “I know that I’ve been called those things multiple times, but I don’t aspire to be that. I’ve always, always said that I’m not perfect. I’m human. I get mad. I get sad. I have all those emotions, but I just like to keep them to myself.”

One person who Gomez did let close to her was her costar Joey King, the 10-year-old who plays Ramona.

“My friends were kind of jealous that I got to hang out with Selena, but I was like, ‘Guys, she’s just a regular person like us,’ ” said King.

The fictional sisters quickly grew close on set and hung out on weekends, having sleepovers and making runs to Starbucks in their pajamas. For her birthday, instead of going shopping with her friends, King opted to have Gomez take her to get her ears pierced for the first time.

“I was like, ‘I feel kind of weird that a 10-year-old has become my best friend, but OK, I’ll go with it,’ ” laughed Gomez.

But in a way, her strong bond with King isn’t all that surprising. After all, the young leading lady is around the age of many of Gomez’s fans.

“I’m fully aware of where I started, and that is Disney Channel,” she said. “They’re what gave me everything, in a way. And I love my fans. They mean the absolute world to me. So whenever I make my music or go with my clothing line or pick roles, it’s all about making sure that they can go see it.”

amy.kaufman@latimes.com