Audiences can see the duo in the new musical romantic comedy "From Justin to Kelly" when it opens this weekend in movie theaters nationally.
Clarkson and Guarini visited The Early Show Thursday to discuss the movie.
In it, Justin (Guarini) and Kelly (Clarkson) are two strangers from different parts of the country, brought together by spring break in Miami.
"We had so much fun making [the film]," said Clarkson. "We did it really fast. There wasn't a lot of time for messing up, so we had to get it right."
Justin and his college buddies from Pennsylvania are party promoters intent on viewing the eye-candy of Florida.
Kelly and her girlfriends from Texas are looking for some sun, fun and relaxation. When Justin and Kelly first catch each other's eyes, sparks fly. But a few obstacles block the infatuation from blooming into a full romance.
"From Justin to Kelly" makes use of its stars' vocal talents and added a few choreographed numbers.
"It's a little different," said Clarkson of making a film. "We're used to stage and musical theater. It's different. The camera is a little more intimate."
Since the two finalists competed on "American Idol," the rumor mill has been working overtime in linking them as a couple.
They insist they are only friends, but the film's audience can see the singers in their first movie kissing scene.
"So [the audience]can just rewind and play," teased Clarkson of the kiss.
As the 2002 "American Idol" winner, Clarkson's star has been rising steadily and swiftly. After signing with 19 Entertainment and RCA, the Texan's debut single, "A Moment Like This/Before Your Love," soared to No.1 in the Billboard charts and broke two world records in its very first week of release.
Her debut album, "Thankful," has met similar success. It sold 279,000 copies and entered the Billboard's Top 200 album chart at No. 1. The 20-year-old Clarkson has since sold more than a million units.
"American Idols" runner-up Justin Guarini released his own self-titled debut album this June.
"I think we wanted to make an album that had more of a broad appeal," said Guarini. "Like Kelly, I didn't want to get stuck in any one thing. Just by looking at our careers over the past year, we've had our fingers in different pots. I think we wanted to keep that up with this album."
Guarini went to college in Philadelphia where he studied performing arts. He later moved to New York to pursue his singing and acting career. Guarini was invited to join the cast of the Broadway production of "The Lion King," but chose not to take the role.
He says his mother told him to compete on "American Idol," and after months of competition, he was in the show's final, but lost to Clarkson. He was, however, immediately signed by 19 Entertainment and given a record deal with RCA.