ADVISORY: This story contains spoilers for the conclusion of HBO’s Mare of Easttown.

“Whatever.”

That’s the exasperated-teen response of Moira Ross in the second episode of Mare of Easttown to her mother’s dictate that she go to the homecoming dance with the date she first accepted, not the boy she wished would ask her.

It’s a small scene that captures the character’s personality in that one word. It’s also a fun moment for Kassie Mundhenk, the Kintnersville, Bucks County, actress who played Moira in the not-always-lighthearted HBO drama, which wrapped up its seven-episode run on Sunday.

Mundhenk, who turned 19 on Thursday, is having a big spring. She recently attended her senior prom, she has Palisades High School graduation on June 11, and since April 18, her friends and family, along with many other people, have finally been able to see her in the role she first auditioned for nearly two years ago.

In Mare of Easttown, created by Berwyn-native Brad Ingelsby and filmed in and around Philadelphia, Moira is the daughter of Lori (Julianne Nicholson) and John Ross (Joe Tippett) and the sister of Ryan (Cameron Mann), all of whom were pivotal in the resolution of the show’s murder mystery. Like Mundhenk, Moira has Down syndrome. And though she appears in one scene in which her character is bullied, Mundhenk’s presence is largely the kind of matter-of-fact inclusion that contributes to the show’s realism.

“I think it was just remembering how I grew up,” Ingelsby said on Thursday of writing a character with Down syndrome. “There wasn’t anyone in my family with Down syndrome, but I just remember being out in the community” and seeing families. “It was something I hadn’t seen in a show in a really long time, and I was interested in portraying a family that had a child with Down syndrome, but never really pointing it out, just having it be part of the community.”

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“It’s really cool to be on TV,” Mundhenk said Tuesday in an interview at her home, where the pictures on the wall include some glamour shots of the teen, who’s been in ad campaigns for the tween clothing store Justice and has been working as a background player — an extra — in movies and TV for about four years. Her mother, Jeanette Mundhenk, said her daughter has appeared in the backgrounds of such series as Law & Order: SVU, New Amsterdam, Manifest, as well as the movies Eighth Grade and Spider-Man: Homecoming.

Moira, though, is her first professional speaking part. Will it lead to other roles?

“I think so,” Mundhenk said confidently.

“You hope so,” said her mother.

“I hope so,” agreed Mundhenk, who has a backup plan, if acting doesn’t work out. “I’d like to go behind the camera to see how it is — or, I would like to work with animals.”

A member of the National Honor Society, she’s had “three years of [studying] TV and video production at the high school,” said her mother, who’s looking for a postsecondary program where Mundhenk can continue her studies in those areas.

During the filming of Mare of Easttown, she attended school in a trailer, as did Cameron Mann, the young actor who played her brother, Ryan. She also had her own trailer, where Mare star Kate Winslet visited her to introduce herself on Mundhenk’s first day on set.

Winslet, she said, was “very fun” to work with and so was Nicholson.

“Julianne bonded with both of the kids” who played her offspring, said Jeanette Mundhenk, whose Facebook page includes some pictures she took during the show’s production as well as a copy of a recent Instagram post from Nicholson’s official account with a picture of the two actresses embracing.

“Anyone need a hug after last week’s episode?” Nicholson wrote. “This girl knows how to give one.”

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Mundhenk also knows how to get what she wants. At her August 2019 audition for the role, she already knew her lines and waved away a script. “Where is she now? Is she already in heaven?” is one she still remembers from the scene she was asked to perform.

“She memorized them in like 10, 15 minutes,” her mother recalled. “I went to get up to go into the room where they were going to videotape it. She turned around and looked at me and said, ‘I don’t need you. I’ve got this.’ ... I just sat back down.”

Others auditioned to play Moira, but “as soon as we saw Kassie, we didn’t need to see anyone else,” said Ingelsby. Nicholson had already been cast, “and I remember thinking, I believe her as Julianne’s kid.”

Mundhenk was also “really wonderful about remembering lines, and she was such a professional ... She was able to pivot off a line if you threw something else at her,” he said. “Her sweetness, that was always written into the character, and then when we saw Kassie laugh, ‘Oh, that’s a no-brainer. Here we go.’”

Jeanette Mundhenk said she’d preferred not to see full scripts of later episodes, to preserve the surprise. So even though her daughter appeared in the finale, the identity of Erin McMenamin’s murderer was as big a shock to both of them as it was to many viewers.

“I was very surprised,” Mundhenk said of the discovery that Ryan, whom her character had visited in a juvenile detention facility, had been there because he had killed Erin.

Said her mother: “She was like, ‘How can my brother do that?’”

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