The 15-year-old had dropped by a friend's autograph session at a Disney Studios event to say hi when suddenly a little girl shrieked her name. Within seconds, hundreds of kids clutching pens and autograph books had surrounded her.
"I was like, 'What! How do you even know me?" says Demi, rising partly out of her chair, tossing up her hands in disbelief and bursting into laughter.
"But yeah," the actress adds quietly, regaining her composure. "It was like, 'This is the coolest thing ever.'"
As the star of the latest Disney Channel original movie Camp Rock -- which the Mouse House hopes will be its next High School Musical -- it's a scene Demi might have to get used to.
"I'm getting kind of anxious, and a little nervous, because there was a picture I saw the other day of Ashley and Vanessa," she says of High School Musical stars Ashley Tisdale and Vanessa Anne Hudgens. "They were at a McDonald's and there was paparazzi all over the place.
"I'm excited, though. For the most part," she concludes.
Only time will tell whether the Disney Channel can capture lightning in a bottle twice in the same decade. Disney is clearly banking on it, however, rolling out buzz about the movie on the Internet for months, offering various Camp Rock collectibles and casting as co-stars the mega-hot pop group the Jonas Brothers. It will air 8 p.m. Friday.
"People are expecting it to be the next High School Musical . That would be an honor if it is," says 14-year-old Alyson Stoner, who plays budding music producer Caitlyn Geller.
In the film, Demi plays Mitchie Torres, a musical prodigy who can't afford to attend a prestigious music camp filled with snooty rich kids until her mother gets her a break on the tuition by landing them both jobs in the kitchen.
Once there, she's befriended by miscreant rock star Shane Gray, played by 18-year-old Joe Jonas. She's also hounded by the rich girls, particularly after she's outed as a kitchen worker, until one of them (17-year-old Anna Maria Perez de Tagle) comes over to her side.
Then, in the time-honored tradition of kid musicals dating to the days of Mickey Rooney and Judy Garland, they bring everybody together for a show at which Demi says happily, "I get to rock the house."
Indeed she does, say the Jonas Brothers.
"She's a great singer. We're writing with her for her new record," 15-year-old Nick Jonas says by phone from Florida, where he was recently on tour with the band.
"And we'll be introducing her on our Burning Up tour this summer," pipes up his 20-year-old brother, Kevin.
As was the case when the first High School Musical film debuted in 2006, most of its teen stars, although unknown to mainstream audiences, already have followings among the demographic of 8- to 13-year-olds.
Demi developed a cult following as one of the stars of As the Bell Rings , the five-minute mini-show Disney uses to fill breaks between its regular programming.
Alyson, who appeared in both of the Cheaper by the Dozen films, voices Phineas' precocious girlfriend in the Disney cartoon series Phineas and Ferb . Perez de Tagle, whose grandmother is prominent Philippine singer-actress Sylvia La Torre, plays the tone-deaf singer, Ashley DeWitt, on the Disney series Hannah Montana .
"And I'm an actual singer!" she says emphatically.
"Being able to finally sing good on Camp Rock is ..." she adds before dissolving into laughter during a recent interview.
There, the actresses have scattered laptop computers, cell phones, schoolbooks and stuffed animals across tables as they rush off to pose for pictures between bites of lunch.
They say they became friends years before their current project, having bumped into one another at auditions and acting classes.
The three say the friendships made filming the movie easier. Even when they had to dance all day long for days on end, they had each other for support, says Demi.
When the movie's producers took them to a remote location in Canada, "where there was no Internet or phone service and there were bears," they still had one another, Alyson adds.