Only three years ago, Kelly Clarkson was shoveling popcorn into jumbo cups at a Cineplex in her hometown of Burleson, Texas. Tonight, nine months after becoming the first American Idol, she pulls up to the same theater in a limo with Justin Guarini, her runner-up and now costar in the movie From Justin to Kelly. The beach musical about two kids falling in love during spring break in Miami--"good, clean family fun," says Guarini--is having a local premiere in front of 300 screaming fans, three weeks before opening on June 20. The theater's salute to Kelly includes a lifetime free pass, plus a block of wet cement where she can immortalize her handprints and signature. As she bends down, her slinky, low-cut dress threatens to reveal a little too much. "Can someone please hold my dress?" she asks. Justin quickly jumps in to help.

It can't be easy, keeping your balance while stoking Idol's star-making machinery, but Kelly and Justin would do Simon, Randy and Paula proud. "Throw something at me, I dare ya," says Clarkson, 21. "Nothing can be as hard as when I didn't sleep for four days and was singing live on TV." She's performing nonstop to promote her platinum album Thankful--"I have the work ethic of a horse"--but thanks to Idol the former popcorn gal now earns more than a few kernels. She is helping her mom buy a house and has splurged on what she describes as a "little vixen" bed for her own Dallas-area apartment.

"It's like the front cover of Victoria's Secret's Christmas catalog," she says. "Red satin."

Guarini, 24, similarly treated himself to a good night's rest with his first post-Idol break, a trip to Hawaii. "It helps me to feel normal," he says. "Because I am. But with all the glitz, you feel like you're in a dream a lot." Who wouldn't? At Michael Caine's and Quincy Jones's joint 70th birthday at Spago Beverly Hills in March, he recalls, "I see Sidney Poitier, I see Billy Crystal, I see Harrison Ford, and I think to myself, 'How did I get here?' These people know my name." With his own self-titled album out June 10, he muses, "Life post-American Idol, wow! Things didn't slow down."

One thing that never got started was an Idol romance. Sure, they hang out together, but contrary to recent reports of canoodling, "Kelly's like my sister," Guarini says. Neither dates--anyone. For Clarkson it's hard enough to keep in touch with relatives by e-mail. "It is a lonely business if you think about it," she says, "so you try not to think about it."

BOX STORY:

Movie Magic: Kelly Gets Klutzy, Justin Is Saved by Mom

From Justin to Kelly "is a full-blown musical that happens to star Kelly and Justin," says director Rob Iscove, who filmed in Miami this winter. "People are expecting an American Idol movie but it's not. It's hip-hop musical theater meets pop rock." But even if it were called Rent, the two names on the lease are Justin Guarini and Kelly Clarkson. A peek behind the scenes:

THE BLOOPERS. "We were rehearsing a scene where we'd just had our first date," recalls Justin, "and Kelly was supposed to open this gate, but she dinged herself in the head with it. And then she tripped on the stairs on the next rehearsal through."

THE COSTUMES. Keep an eye out for Kelly's far-out skirt, designed by Bobbie Read: It's made of ties. "It was really weird," she says, "and I was like, 'I want to keep that!' So I did."

THE PERILS OF LOCATION SHOOTING. Kelly says the outdoor temperature "was like 40 degrees" for a scene that required Justin to fall into a pool. "I had to keep jumping into the water and jumping back out," he says. "I was trying Zen-breathing exercises to keep from freezing."

THE FANS. Between takes, Justin hid his curls under a ski cap to wander around incognito. But it didn't work. Spotted by some teenage girls, he stopped to sign autographs until his mom, who was visiting, summoned him back to the set. "I've got to go when my mom calls," Justin said sheepishly.

AFTER-HOURS. Karaoke at the Shelborne Beach Resort. "Justin is singing 'Let's Get It On' with that beautiful voice," says costar Brian Dietzen, 25. "You haven't lived till you've done karaoke with an American Idol."

COMING ATTRACTIONS. Both are concentrating on music, but why rule out future roles? Guarini says he has a standing offer to play the emcee in Broadway's Cabaret. Says Kelly: "My dream role is like Kathy Bates in Misery. Or Dumb and Dumber!"