For a 22-year-old who has been world-famous half her life, Emma Watson is refreshingly unaffected. Her decade-long role as Harry Potter's know-it-all pal Hermione Granger subjected her to child-star stresses that can only be imagined. She was working on a high-stakes franchise since she was 11 and a subject of public curiosity at an age when most kids can blunder in relative anonymity.

Yet she's emerged a charming, well-spoken, self-possessed professional with all the brainpower of her signature character and a bright sense of humor besides. Lindsay Lohan's antimatter.

In her first role since completing the Potter series, Watson signed on for the film adaptation of the young-adult bestseller "The Perks of Being a Wallflower," directed by its author, Stephen Chbosky.

She plays Sam, a 1990s high school senior whose friendship means the world to introverted freshman Charlie (Logan Lerman). A voracious reader, Watson said she prefers to act in films based on novels.

"I like books a lot. It gives a lot more depth and detail that you sometimes don't really get with a script. I guess I'm just lazy as an actress and I like to have all the work done for me," she said with a giggle. "It's nice to do something with a whole world that's created and padded out for you," and she regularly came to the film's Pittsburgh sets with a heavily marked up copy of Chbosky's novel in hand.

Acting the part of a public school student was a stretch for Watson, who grew up learning from private tutors, and the greatest company of British film actors ever assembled. But her youth wasn't that different from the high school world of cliques and adolescent uncertainty, she said. "Have you ever been on a film set? It's just a different pack of wolves," she said.

Growing up in the spotlight has been "great in the sense that no one can throw anything at me that will surprise me. It's given me the stamina, the knowledge and experience I need to go ahead being very confident. If there were a downside, it would just be that all of that was on film. I had to learn on the job. All of the stuff that people usually do behind closed doors, I've done on camera. It makes me a little touchy and twitchy at times. The fear factor has been a disadvantage, the awareness that a lot of eyes were on me."

Watson has not insisted on stepping directly from the ensemble films of her youth into leading roles. She recently filmed "The End of the World" playing herself alongside Jason Segal, Jonah Hill and a slew of other celebrities who face the apocalypse while attending a party at James Franco's house. In Sofia Coppola's true-crime story "The Bling Ring," she's part of a girl gang who use social media to track celebrities' whereabouts and burglarize their homes.

"I'm not so much picking roles as picking directors, and stories as a whole that I really care about," she said. "I wanted to make this book for a line that my character doesn't even say: 'You accept the love you think you deserve.' Paul Rudd and Logan get that line. But I cared about the pieces and really wanted other people to see the movie."

To master the nuances of Sam, she e-mailed Chbosky "hundreds of times right up to shooting, saying, 'I don't get this joke. What does this slang really mean? What is an Olive Garden?' I was really nervous. I knew that Logan and Ezra [Miller, who plays Sam's half-brother] grew up in American culture. They had an experience they could draw on. Not only did I not have the experience, I was doing an accent. So I felt like I was 20 steps behind and pedaling fast to get where everyone else was."

In one memorable scene, Watson stands in the bed of Miller's pickup truck, arms widespread, wind whipping her hair as David Bowie's "Heroes" blares in the tunnel they are speeding through. She had to argue for the opportunity to do it herself, tethered to the vehicle by a cord as it sped along.

"The stunt team weren't too happy about me doing it. I had to fight for it. As a kid on the Harry Potter films they treat you like a piece of china. As a kid, that's so horrible 'cause all you want to do is throw yourself at stuff, do dangerous things, play sports and run around. The back of the truck felt so beautiful and freeing. I have developed quite an aversion to being treated like a piece of glass, so that experience was awesome."

Colin Covert • 612-673-7186