Watson was in this leafy suburb of Pittsburgh, a 30-minute drive from the glass-and-steel downtown, filming a movie that’s set at Peters Township High School. Every day she arrived at the sprawling campus, with its swim team and banners promoting reading, to experience the youthful rites that, as the Oxfordshire-bred star of the “Harry Potter” franchise from age 10 to 20, had otherwise eluded her.

“Oh my goodness, so many firsts,” she said, speaking in an excited rush during a break from filming. “I did the prom! We all get dressed up and we go in a limo, and get photographs. It’s been really fun for me to get to graduate. Eating in the school canteen; all these things that I’ve always sort of said to my American friends, ‘Oh, that looks amazing, that looks so fun, I’m jealous.’ And I get to do it for this movie.”

And Watson, accustomed to acting alongside people and in a story she had known for a decade, “started freaking out,” she said. She threw herself into research [for the 'Perks' role], e-mailing friends about their high school experiences and worrying about how to create a bond with this new, unfamiliar clique.

“That’s a different chapter of my life, which, kind of through doing this, feels like it’s closed,” she said. She pointed to a scene in “Perks” as symbolic of her new beginning: standing in the back of a pickup truck, she and her high school crew take a late-night joyride through the Fort Pitt Tunnel, the city lights shining on the other end.

“Summit didn’t want me to do the stunt, but I insisted,” she said, even though she was scared. “The car’s moving at 60 miles per hour, I had one little thing attaching me to the truck,” she recalled. She ended up going through seven or eight times, screaming her guts out. “Oh my God, it was so fun,” she said. “One of the best nights of my life, without a doubt.”