“Line?”

Alanna J. Smith, decked out in emerald green face paint from head to toe and a witch’s black hat, yelled frantically to the stage manager.

It was holiday season in 2016 and Smith was almost finished with her first staged production of The Wizard of Oz at the Walnut Street Theatre, having made zero mistakes so far.

The line that slipped her mind? “I’m melting.”

“Of course, the most famous line is the one I miss,” said Smith.

Not to worry. Smith, now an in-demand musical theater actress, was at the time an understudy for the Wicked Witch of the West role, and her flubbed line came during the “understudy run” of the production — a tradition in most Philadelphia theaters.

For the understudy run, the entire cast of understudies performs on stage in one full production, shortly after the regular cast’s opening night. The audience consists of friends, family members, and artistic scouts.

Smith’s wobble in the Wizard production didn’t hurt her career: She’s recently done Noises Off at the Walnut, Grease at the Fireside Theatre in Wisconsin, and she goes into rehearsal next week for Ken Ludwig’s A Comedy of Tenors, opening Jan. 15 at the Walnut.

The private, family-and-friends production is just one aspect of the secret life of understudies on the Philly theater scene, those largely unsung actors who stand ready to stand in on short notice for others.

A way to get noticed

Jonathan Silver, associate producer at the Arden Theatre, says many are fresh out of acting school. Casting them for the often thankless stand-in role gives producers and directors an opportunity to learn their style and work before they’re cast in larger roles.

On any given production night, a couple of dozen actors are poised to go on at the last minute for shows across the region. It’s a chance for an actor to get identified as a talented new artist. But it’s a very different discipline from acting as a regular role.

Understudies might have only 20 hours of formal rehearsal, compared to the regular actors who practice for about three weeks before opening.

It’s up to the them to stay on top of their game and go see the show repeatedly from the audience — admission is free for them — and to read, reread, and rereread the script on their own.

“You really have to rely on your own brain,” said Georgiana Summers, a junior at the University of the Arts who was an understudy in the spring as Molly in Theatre Horizon’s Peter and the Starcatcher. “Instead of rehearsals, I was on my couch tearing my hair out trying to learn the lines and also the accents.”

Summers has just been cast in the forthcoming Philadelphia Theatre Company production of The Bridges of Madison County (Feb. 8-March 3), where she’ll play Francesca Johnson’s daughter, Carolyn.

Like Nick Foles, minus the millions

The role of an understudy can be compared to that of a backup quarterback, like Nick Foles. And the odds are long that they’ll ever get their Super Bowl call-up.

Silver estimates that 95 percent of the time over a season, the regular cast will go on with no need for an understudy.

Still, understudies’ night lives are on hold during the full run of a show because actors can get sick, injured, or called out on emergencies without warning. Understudies are asked to stay within a 30- to 45-minute radius of the theater during showtimes and can be called in as late as halfway through Act One.

Because they’re not paid for much stage time, it’s extra-critical for these budding actors to keep their day jobs.

Rachel Camp, now a Barrymore Award-winning musical theater actress, supplemented her understudy days as a theater teacher and continues to teach occasionally for the Arden Theatre Company. Summers has supported her acting habit with summer jobs making puppets in her hometown of Frederick, Md.

Lindsay Mauck, an actress and understudy, has both a “passion job” as the managing director and producer of Mauckingbird Theatre Company, specializing in gay-themed productions, and a day job as managing director of Philadelphia Young Playwrights.

‘A different muscle in your brain’

On top of worrying about when they might fit into the show, understudies grapple with another big question: how?

“It just flexes a different muscle in your brain,” said Camille E. Young, a professional actress who has at times played understudy and swing roles (a cast member who covers vacation days and other vacancies for several roles at once). She is currently a swing in four roles for The Color Purple at Theatre Horizon.

For one thing, Young said, you have to memorize blocking and stage directions rather than practice them repeatedly.

Another challenge is that understudies are expected to replace the featured actor so seamlessly that the rest of the cast doesn’t even notice. So instead of making their own creative calls and choices, understudies learn to blend in.

“It’s important to know how to imitate — imitate is a strong word — the choices of another performer,” Summers said. “You have to still be genuine and bring your own life to the role while keeping through what another performer already created.”

Mauck said the task of being an understudy “is not for the fainthearted.”

“It’s a fine line between bringing your personal touch to the role and honoring and respecting everything the person in the role has done before you,” Mauck said. “You have to do it in a way that keeps everyone else in the show comfortable and at ease performing with you.”

A secret sense of relief

The most surprising secret of the lives of understudies may be that they’re not necessarily upset when they don’t get the call.

In the full run of Peter and the Starcatcher, Summers never went on. To her, that wasn’t a big loss.

“To be honest, it was kind of relieving,” she said. “There was a very big part of me that never wanted to go on stage.”

Nichalas L. Parker started understudying at the Arden Theatre immediately after graduating from the University of the Arts, then got an apprenticeship at the Walnut Street Theatre.

He said that “luckily,” he was never in a position to replace someone on stage when he understudied. He was conflicted about whether he genuinely wanted to get on stage and perform.

“There’s a mixed feeling between stress where it could happen at every moment, where you’re constantly checking your phone for someone to be like, ‘Hey, we need you on stage in the next five minutes,’ but then also wanting to get on stage and show what you can do,” said Parker.

Parker has gone on to leading roles in musicals ­– including the Lion in Walnut Street Theatre’s The Wizard of Oz – and will play an as-yet unannounced starring role in one of theater’s forthcoming mainstage productions.

“Being an understudy,” said Parker, “pushed me forward.”

Twice as nice

Camp, now considered one of the top actresses on the local scene, made a big impression as an understudy in 2010 for Sondheim’s Sunday in the Park with George at the Arden.

She’d been cast as an understudy for not one, but two, roles. And they were twins.

When it was time for the understudy run and Camp had to choose between Celeste #1 and Celeste #2, she couldn’t decide.

“It felt impossible to do one of the twins, so I played both,” said Camp.

She sang and bickered back and forth to herself as both roles simultaneously. She described it as an “absurd and entertaining” exhibition.

Though there weren’t many people in the audience, associate producer Silver was.

“Just as a performance, it was absolutely brilliant,” said Silver.