When Justin Guarini was announced for the role of Carlos in Women on the Verge of a Nervous Breakdown, his Broadway debut, the coverage focused on his participation in the inaugural season of "American Idol," in 2003, when he was runner-up to Kelly Clarkson. Less noticed was the fact that his student training was richly theatrical.

"I studied vocal performance, dance, acting, and theater in college," Guarini said the other day after rehearsal. "But the theater part had started in high school. I was a little bit of a late bloomer." He started by playing Hortensio in The Taming of the Shrew, and went on to other lead roles in The Pirates of Penzance and The Mystery of Edwin Drood.

"My high-school roles were very cool, and a little offbeat," Guarini said. "We weren't doing the big standards like The Music Man and My Fair Lady."

Guarini's first brush with Broadway came when he was a student at "The University of the Arts," in Philadelphia. (He spent most of his childhood north of Philly, in Bucks County.) While there, he took part in Philadelphia auditions for The Lion King.

"They were interested in me for Simba," Guarini said. "He's buff, to say the least. But I wasn't right for that role. I continued to audition, and did master classes for The Lion King."

Four or five years later, The Lion King casting called. "They said, we have a chorus role for you on Broadway," Guarini said. "I told them I would love to do it, but I was going out to L.A. to audition for 'American Idol,' which no one had ever heard of at the time. I said I needed a week or two before deciding, because I didn't know if I'd make it through the next round of 'Idol.' They said: 'We'll give you a week.' So I had to decline, regretfully."

Needless to say, Guarini made the right career move. "American Idol" became an instant sensation, propelling him into the public spotlight. "That first season made an especially big impact," he said. "It cut a huge swath through every demographic in America." He added: "I'm very grateful for everything 'Idol' has given me." Even the public exposure? "I'll be worried when people no longer recognize me," Guarini answered. "I just had to train myself not to see people clocking me when I walked down the street."

"American Idol" helped give Guarini a recording career (he's currently working on his third CD, which, he said, "will be more of a statement of who I am"), and led to various "Idol"-related projects. He's gotten involved in charity work, especially for music education, and proved himself a charming, capable TV host for the TV Guide Network, working the red carpet for the major awards shows. He's also done movies and television.

"Public speaking is in my blood," Guarini said. "My father [Eldrin Bell, the current Chairperson of the Clayton County Commission in Georgia] is a politician. My mother [Kathy Pepino Guarini] was an anchorwoman for CNN. My stepfather [physicist Jerry Guarini] has also done his share of public speaking."

Guarini said that coming back to theater is both scary and exciting. "It's a little frightening, because in concert I'm often acting as a solo performer with the audience. For Women on the Verge, I'm part of a larger group of performers." He added: "I'm having to reacquaint myself with that group dynamic. But this is the first time in years that I've felt challenged. I'm in this sea of extremely talented, extremely experienced theater people. I can't just sing my way out of a challenge. I'm feeling extremely vulnerable, but it's such honor to share the stage with this caliber of castmates."

After three weeks of rehearsal, what has Guarini learned about his character, Carlos? "He's a little nerdy and awkward. He's not one to explode under pressure, but there's this constant undercurrent of intelligence and emotional turmoil begging to come out. My biggest task is to contain all that."

BRENDAN LEMON is the American theater critic for the Financial Times and the editor of lemonwade.com.