Idina Menzel has the actor's disease: Though she enjoyed huge success in the original Broadway casts of Rent and Wicked - and now on the hotter-than-hot Fox series Glee - she's haunted by the possibility of never working again.

That notion was particularly intense after the 2009 birth of her son. "I was afraid people would forget about me," she said the other day. "I live in Los Angeles much of the year. I'm surrounded by huge movie stars, so the perspective is out of whack."

Few of her talented Broadway colleagues graduate to the concert stage - particularly ones as large as the Mann Center, where she'll be on Thursday with the Philadelphia Orchestra.

It's a sweetheart gig. The evening is hers - she's nobody's guest star in a larger program - and that suits her expansive personality. Menzel believes that the more she sings over a day or week, the better she sounds. She has not only her Wicked signature song "Defying Gravity" to draw from, but three solo albums.

That may not quite add up to being on top of the world, but she shows every sign of doing pretty well. In fact, she inspires enough scrutiny from fans that blogs abound with speculation of a feud with Wicked costar Kristen Chenoweth that apparently has no basis in fact.

All good news? "On a day-to-day basis," said Menzel, now 39, "I don't see it like that.

"I do have these albums out and I've been trying so hard to cross over into a pop mainstream kind of thing. But I still sell mostly to the theater audience. Glee helps in some way but not drastically. Not yet. I've had a lot of disappointments with my own albums and sales."

She says her continued presence on Glee is by no means assured. She plays the director of a rival high school glee club, Vocal Adrenaline, and in the season-ending cliffhanger her character is holding someone else's baby while trying to establish a relationship with the teenage daughter she gave up for adoption. How could that plot line not continue?

"They're very secretive over there," says Menzel. "I'd love to come back. It's great to be in a show on TV where they really appreciate and respect theater people."

But -

As insurance against unemployment paranoia, she's in New York workshopping a new musical, details of which she can't discuss. "It's important to make new projects," she said in a phone interview between rehearsals.

And then there are concerts: "The reason I love touring so much is that it gives you control over a career that normally has little control."

Unlike many Broadway performers, Menzel has no problem holding the stage without character or plot. Though she has theater in her bones, most of her early training on her native Long Island was that of a wedding and bar mitzvah singer.

"I've taken voice lessons since I was a little girl. Here I was, a 15-year-old girl listening to all kinds of songs and learning swing, jazz, and all the pop music of the time, including Motown. And you have a platform where you can try things . . . one where, frankly, most of the time, people aren't listening. I'd emulate Ella Fitzgerald and Sarah Vaughan in the car driving to the show, and then sing some jazz standard and make it my own."

Her break came in her mid-20s with Rent, which was mostly cast with non-theater types. The show started in a small, nonprofit, Off-Broadway venue, and within a few months was a raging, on-Broadway hit, one of the few to attract the kind of young, East Village artistic denizens portrayed so vividly in Jonathan Larson's score.

What came after explains why she fears not working: Some seven years separated that show from her bigger, 2003 breakthrough in Wicked. In between, she had occasional work. "But I couldn't get a gig, got dropped from my record label, and thought Rent was that special thing that happens once in lifetime," she said.

However, she has something many performers don't have, and that's her own creative life. She's been writing her own songs for years with pungent, keenly observed lyrics - one reason the concert stage is nothing alien to her. "I'm comfortable with being me. I enjoy spending time with an audience, telling them why I wrote a particular song, why I'm performing a particular song," Menzel said. "When I go to a concert, I like to feel that the performer is being real, even if it's banter that they say every night."

Real is not a problem for her. Thanks to her garrulous streak, she even had a talk-show appearance in which she discussed getting through labor without screaming "the f-word."

"I did?" she asked incredulously.

It's on YouTube.

She also thanks people compulsively, whether she's accepting a Tony Award or introducing members of her band. "I honestly don't know how to be anything else. It might get me in trouble sometimes," she said. "Sometimes my husband winces."

That would be the handsome star of Private Practice, Taye Diggs. Both were in Rent in the mid-1990s, and they married in 2003. He's now an important foil in her real life. "We pick up where the other leaves off. My husband loves to go out and dance and do nightlife. I can be a homebody," she says. "So it's nice to have somebody who drags me out of the house and literally gets me dancing."

Her dark, intense looks suggest why she's cast in edgy parts, such as the commanding bisexual performance artist in Rent or the dark-garbed, green-faced witch in Wicked. Possibly her best stage role was the darkest of them all: the Off-Broadway Michael John LaChiusa musical See What I Wanna See, with its film noir-ish plot about scorching sex, murder, and elaborate mystery.

"I'm dying to get back onstage here, but I don't want to do just anything," Menzel said. "I'm such a specific type that I don't fit into a period musical. I want to find something that means something. Something that scares people."

Idina Menzel and the Philadelphia Orchestra

8 p.m. Thursday at the Mann Center, 5201 Parkside Ave. Tickets: $10-$50. 215-893-1999 or www. manncenter.org.

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Contact music critic David Patrick Stearns at dstearns@phillynews.com.