NORTH BEND — No one asked Tyler Kemp to understudy all 55 actors in “The Wizard of Oz.”

As a Munchkin who doubles as a flying monkey, the 9-year-old doesn’t spend much time onstage in the play, which opens tonight at Little Theatre on the Bay.

The other Munchkin generally go home after rehearsing their scene. But Tyler’s mom and sister appear in a scene near the end.

So he stays all evening, watching and whispering almost every line before the actors on stage say them.

That’s what happens after so many weeks of rehearsals, according to Alyssa Birrer, who stars as Dorothy: “After a while, you know the whole show.”

With this play, though, it’s not just the actors who know the lines. Because the script closely matches the classic film, the cast can expect audiences who know the show as well as they do.

“Everybody knows the movie by heart,” said director Byrell Justice. “If they don’t hear those lines, they feel ripped off.”

The biggest responsibility falls on Birrer, who is on stage almost continuously for three hours, appearing in all but two scenes. It’s something she has wanted to do for a long time.

“She’s that girl that everybody wants to be,” said Birrer, 17.

Birrer said her life changed when she saw “Wicked” on Broadway a few years ago. The show sparked her interest in musical theater.

“Getting to play Dorothy, it’s like coming full circle,” she said. “It’s very rewarding.”

There are times when moving from an exit to her next entrance requires Birrer to go outside the theater building, something she especially enjoys when children are passing by.

“Just seeing their faces light up when they see my costume — ‘There’s Dorothy!’ — that’s pretty cool,” she said.

Though the dress is recognizable, it’s not the same as what Judy Garland wore. It’s designed to make Birrer look much younger — more like the 11-year-old Dorothy from the book, rather than the 18-year-old Garland.

Being true to L. Frank Baum’s novel was one of Justice’s goals for the show. Replicating the movie on stage would be impossible, he said.

“It’s like trying to be Babe Ruth,” Justice said. “Lightning doesn’t strike twice. You gotta find a way to make it your own. If you try to imitate Bert Lahr and you don’t sell it 150 percent, they’re gonna hate you.”

In Birrer, though, Justice got an actress who sounds more like Judy Garland than anyone expected, delivering each line with a distinct warble. Birrer said she has spent hours working with Justice, and she credited the many veterans in the cast with helping her develop her performance.

There are some departures from the movie, particularly the inclusion of deleted scenes. As the Wicked Witch of the West, Cassie Justice gets several funny lines that make her a more likable character than Margaret Hamilton was allowed to be.

There’s also the dance of the Jitterbugs, which features Tyler’s mom and sister, Angie and Alyssa Kemp, and keeps Tyler in his seat late into the night.

As the play goes on, he also notices when somebody’s out of place. So perhaps the director’s job can be added to those the Blossom Gulch Elementary School fourth-grader is prepared to fill. If there’s one he has his eye on, though, it’s Alexander Rich’s part — the floppy, comical Scarecrow.

“Because he gets to roll a lot,” Tyler said.

About the play

When a tornado whisks away her Kansas farmhouse, Dorothy Gale (Alyssa Birrer) finds herself in the land of Oz. Though the native Munchkins (see box) celebrate her arrival, she wants to return home to her Auntie Em (Joyce Kemp) and Uncle Henry (Jim Kemp).

To do that, according to Good Witch Glinda (Genna Dunning), she’ll have to go see the Wizard (George Nixon). Along the way, she meets a Scarecrow (Alexander Rich), Tin Man (Tim Novotny) and Cowardly Lion (Jeff Roberts), who help protect her from the Wicked Witch of the West (Cassie Justice).