From left: Erin Weaver as Sally, Michael J. Mainwaring as Schroeder, Christopher Michael Richardson as Charlie Brown, Harrison Smith as Linus and Awa Sal Secka as Lucy in “You’re a Good Man, Charlie Brown” at Imagination Stage. (Margot Schulman)

Sure, a teenage brother or sister can mimic a much-younger sibling with a high-pitched whine or a scaredy-cat shriek.

But that’s overacting, and it’s strictly for amateurs.

When professional actors play child characters, they try to simplify and heighten, not exaggerate. It’s an art and kind of a science, too.

“If you’ve seen any kid playing in the yard — pirates or ‘Star Wars’ or whatever — they make up a scenario in their heads and they completely believe it,” says 21-year-old Carlos Castillo, who plays Peter Pan in “Tinker Bell,” a new play by Patrick Flynn at Adventure Theatre. “Being actors, especially doing [theater for young audiences], is getting back to that childlike state.”

So, where does an adult actor playing a kid start on Day One of rehearsal?

We sought the perspectives of Castillo and four other adult actors now playing children, or childlike characters, on local stages.

“All the complexity that you have as an adult, that people find interesting onstage as an adult, that has to go away, the first layer, before you can figure out who they are as a person, how they operate and how they take in the world,” says Christopher Michael Richardson, 27, who came to the title role in “You’re a Good Man, Charlie Brown” at Imagination Stage straight from playing the Lion in “The Wiz” at Ford’s Theatre.

Erin Weaver, who plays the Peanuts character Sally Brown, says she was always a “big studier of children.” She noted that “when your character’s angry, this is the one thing that they do. When they’re happy, this is the one look.” (Margot Schulman)

Erin Weaver, a five-time Helen Hayes Award winner, glides among classical and contemporary plays, musicals and shows for young children. As Charlie Brown’s kid sister, Sally, she’s supposed to be 4 years old, and that, says the 38-year-old, has equally to do with physicality and mind-set.

“I’m a big studier of children,” she says. “I used to teach children for years before I had my own [6-year-old Maisie, with her husband, “Charlie Brown” director Aaron Posner]. And I just feel like I pick up their behavior and mannerisms, and also the way they think and see the world.”

Little ones, Weaver notes, tend to feel only one emotion at a time. Distilled for the stage, that means that “when your character’s angry, this is the one thing that they do,” she says. “When they’re happy, this is the one look. When they’re sad, this is the one look. And you make sure that you’re always moving very crisply between those three or four emotions.”

That, too, is how author J.M. Barrie described Tinker Bell: “Fairies have to be one thing or the other, because being so small they unfortunately have room for one feeling only at a time.” Michelle Polera, 31, runs with that while starring in “Tinker Bell,” which tells the “Peter Pan” tale from Tink’s point of view.

Carlos Castillo as Peter Pan and Michelle Polera as Tinker Bell in “Tinker Bell” at Adventure Theatre MTC. Polera also understands kids’ simple emotions: “If my character was responding to something bad that happens, she was angry about it.” (Bruce Douglas)

“That’s kind of how I attacked the role,” she says. “If my character was responding to something bad that happens, she was angry about it. If something happy happens, she felt joyous.

“The movements and the actions that I have I can say are from experience of working with kids,” adds Polera, whose mother ran a day-care center out of their home.

Emily Whitworth, 23, actually went pro as a 12-year-old, acting in shows at Shakespeare Theatre Company and Studio Theatre, and she started at Synetic, with its choreographed theatricality, while in college. For Synetic’s “The Wonderful Wizard of Oz, she says she wants her Dorothy to seem childlike but of no specific age, with “no sense of irony or sarcasm” and a “profound innocence.”

Emily Whitworth as Dorothy in Synetic Theater’s “The Wonderful Wizard of Oz.” (Johnny Shryock)

Dorothy gets no between-the-lines subtext, either. “So many characters, their subtext is sometimes stronger than their text,” Whitworth says. “Playing teenagers, everything they do is subtext. Everything is ironic, everything means a different thing. But for Dorothy, what she says is what she feels.”

The biggest reward from playing a child is seeing the reactions from the kids themselves.

“When I don’t hit the ball, then I sort of sulk offstage, we had one boy scream out, ‘We love you, Charlie Brown!’ ” Richardson says. “It was the most adorable and wonderful thing.”

Polera’s Tinker Bell has the closing line: “I say, ‘Do you believe in fairies? You should, because I believe in you.’ And after I said that, there was this little boy that just screamed, ‘Thank you!’ That made my day.”

If you go
You're a Good Man, Charlie Brown,

Imagination Stage, 4908 Auburn Ave., Bethesda.

Dates: Through Aug. 12.

Tickets: $12-$30.

The Wonderful Wizard of Oz

Synetic Theater at Georgetown University’s Davis Performing Arts Center, 37th and O streets NW.

Dates: Through Aug. 12.

Prices: $20-$45.

Tinker Bell

Adventure Theatre MTC, 7300 MacArthur Blvd, Glen Echo.

Dates: Through Aug. 19.

Tickets: $19.50.