American idle: Kelly Clarkson tries to maintain momentum.(Knight Ridder Newspapers)

Byline: Douglas Perry

HOLLYWOOD, Fla. _ On a bright February morning, Kelly Clarkson steps onto the beach in a black one-piece swimsuit and a sheer ankle-length purple skirt. She is plainly overdressed.

Beautiful college girls as close to naked as possible stand around in twos and threes, preening for their next boyfriend or just gazing out at the stark blue of the sea. But this party-ready bikini brigade isn't here on spring break. They're extras on Clarkson's first movie, "From Justin to Kelly: The Rise of Two American Idols." And they have a very specific mission: to ratchet up the sex appeal so that teen-agers, and not just "tweens," head out to the multiplex to see the film.

Clarkson is happy to take all the help she can get. She knows as well as anyone how much depends on this ambitious musical comedy, co-starring her "American Idol" runner-up, Justin Guarini.

After all, it's been five long months since the 20-year-old Burleson, Texas, native was crowned "the next American Idol" on national TV and her single, "A Moment Like This," hurtled to the top of the charts. Since then, the November release date of her CD has been scrapped and she's dropped off of "Entertainment Tonight's" radar screen. A month ago, "American Idol 2" began _ promising a new "idol" by May.

Worried that Clarkson's "American Idol" triumph could be slipping from teens' minds, the filmmakers admit they're rushing. "From Justin to Kelly," which everyone on the set reflexively describes as "`Grease' on the beach," began shooting in January and is scheduled to reach theaters in April _ an astonishingly tight time frame.

"We've definitely been pushing it," says Jennifer Radzikowski, the film's location manager. "There were a couple of nights when I sweated it out, thinking, `Oh my God, what are we gonna do?' But we had to do it."

If talent was all that was needed, Kelly could afford to relax a little, take a couple of days off and just lie around on one…