Underwood was their choice for Maria "from Day One," says Meron, citing her personal similarities to the religious character and, of course, her comfort with performing on live television. Underwood's acting experience, however, is so far limited to a guest spot on "How I Met Your Mother" and a supporting role in the inspirational film "Soul Surfer."

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But even her scant IMDB résumé wasn't cause for much concern, according to Meron. "We've trusted our instincts in the past in terms of making surprising casting choices." (Case in point: Everyone in "Chicago.") 

The fact that no one has staged a live television musical in nearly five decades meant that Meron and Zadan have been working without a map. Early on, however, they determined the project would require two directors. In addition to McCarthy-Miller, who worked on "Saturday Night Live" for 11 seasons and also directed both live installments of "30 Rock," they enlisted Tony-winning director and choreographer Rob Ashford, who spent several weeks at a rehearsal space in Tribeca working with the cast on their performances.

For other decisions, the producers relied on department heads with wisdom gleaned from years in the theater and opera. Production designer Derek McLane determined a layout for the show's seven sets that made sense aesthetically without requiring cast members to run hundreds of yards between scenes. Costume designer Catherine Zuber, meanwhile, was tasked with creating costumes that were period-appropriate but conducive to quick changes and live singing. (For example, she had to sand down the fabric on the nuns' wimples so they could hear the music.)

If all goes well on Thursday, Zadan and Meron hope to spark a new holiday tradition and bring live musicals to television every year. 

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Laura Benanti, a Broadway vet who plays the Captain's scorned fiancée Elsa, applauds their efforts to democratize the medium, which, for many Americans, is financially and geographically out of range.

"Musical theater is not something that we should just let fade into the background," she says. "It's an American tradition — apple pie, jazz, baseball and musical theater."

As zero hour approaches, everyone has his or her own particular set of concerns and methods for combating anxiety.

McDonald acknowledges that she's worried about getting through "Climb Ev'ry Mountain" without being overcome by emotion. Moyer predicts he will "go completely OCD" making sure all his props are in place. Underwood plans to "be praying extra hard."

This being a one-time-only live broadcast with no understudies, what worries producers more than anything is the wintry weather ahead. As a prophylactic measure, the craft services table is equipped with enough hand sanitizer and vitamin C to protect an entire Alpine village, and a doctor will be on duty the day of the broadcast.

"What if someone gets a cold and gets laryngitis? You're panic-stricken if someone gets the flu," Zadan says. "Those are the things you never think about when you're shooting because you can just record it some other time. When you're doing it live and it's once and it's one night … you hold your breath."

meredith.blake@latimes.com  

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'The Sound of Music Live!'

Where: NBC

When: 8 p.m. Thursday

Rating: TV-G (suitable for all ages)