"I'm used to singing in a character voice, being told, 'Do it with an accent, or do it in a '40s style,' and with a wig, or a dialect. When they say, 'We want to hear you,' I think, 'What do you mean? I imitate people, that's what I do,'" Shields said. "You feel naked."
Of course, anybody might feel a little nervous about singing duets with both Esparza and Kendra Kassebaum (an admired Glinda in "Wicked"), and finding a place among theater veterans, who, as Shields put it, "sleep, eat and breathe this stuff."
She told her costars from the outset, "'Look, I'm going to try to steal from you, because you're the best.' And they said, 'Well, what do you think we're doing out there every day?'
"It's hard to find your own sound, not being a recording artist," she went on. "For example, Raul has a very specific, amazing sound and range and style. He'll say, 'Today I'm just going to "mark it" [musical theater slang for 'just go through the motions'] and I'm, like, 'Your "marking it" is the rest of the world's top-notch performance.'
"It's very all-encompassing, but I actually have a life that I have to get back to. Very inconvenient." Shields sighed. That life includes her husband, Chris Henchy -- a comedy writer for, most recently, the film "The Other Guys" -- and two young girls, 7 and 4, who had just recovered from colds that made Shields fear for her already-taxed vocal cords as she snuggled them to sleep.
She enjoys exposing her daughters to the magic of behind-the-scenes theater and returns to it herself almost as a form of training.
"The work ethic in the theater is so refreshing. I need to have a dose of it from time to time.
"If you're lucky enough to be in this kind of environment, with this amount of talent surrounding you all day long, you've got to keep your eyes and ears open, and be ready to be inspired by it and changed by it."
The iPhone lighted up, and Shields took the call. "They said they'll call me in an hour to tell me when they're going to call me." She smiled at the hovering waiter. "Could I have a double espresso?"