Teen Mckenna Grace on ‘A Friend of the Family’ predator: I wanted to ‘vomit from how gross it is’

Mckenna Grace faced the challenge of separating art from life in “A Friend of the Family,” based on a true story with horrific overtones.

The nine-episode drama, ending its run Nov. 10 on Peacock, unravels the real-life story of Jan Broberg, who was groomed and kidnapped — twice — in the mid-’70s by pedophile Robert “Bob” Berchtold (“The White Lotus” Emmy nominee Jake Lacy) once he charmed his way into her tightly-knit Mormon family in small-town Pocatello, Idaho.

Grace, 16, plays the teenaged Jan, with Hendrix Yancey portraying Jan as a younger child. Anna Paquin and Colin Hanks play Jan’s trusting, church-going parents, Mary Ann and Bob Broberg, who initially fall under Berchtold’s evil manipulative spell — but, as the series progresses, fight at all costs to get their daughter back from the predator’s creepy clutches.

“I’m reading [the script excerpts] in the morning and I’m like, ‘Oh my God, [Lacy/Berchtold] has to say that to me and I have to not cringe and just vomit from how gross it is,” Grace told The Post. “I had to block every instinct in my body that was telling me, ‘Please, run away, this is horrible,’ because Jan really was in love with him and she never thought he would ever try to do something bad to hurt her or her family.”

The married-with-kids Berchtold (Lacy delivers an uber-creepy Emmy-worthy performance) sets his eyes on 9-year-old Jan when the two families first meet and slowly starts to insinuate himself into her life. Eventually, he convinces her that she’s a half-alien from a distant, dying planet who’s being watched over by beings named “Zeta” and “Zethra” — and that, if she doesn’t marry him, have his child and complete “The Mission,” her family will be irreparably harmed.

“To Jan it was so real and they were trying to protect each other and have an alien child and save the planet and her family,” Grace said. “So, going into it, I just had to say, ‘OK, I’m in love with this man right now and that’s how I’m playing this.’

“Mr. Jake is just fantastic and made me feel so comfortable,” she said, referring to Lacy, “because portraying a relationship like that, it’s really important to have trust with your scene partner and to make sure everything is comfortable and safe for both of you as actors. We had an on-set counselor to help with the weird, intimate parts of the relationship.

“I was solely playing off whatever [Lacy as Berchtold] gave me because that’s what Jan’s life was at the time,” she said. “She was listening to the inflection of B’s voice to see if he was upset or not, or hanging on his every word hoping to hear from Zeta and Zethra … he’s wanting her to go from being afraid of all of this to ‘You are also are my wife and you love me … like your husband.’

“He’s already so far into the brainwashing [of Jan] and now he’s turning their relationship to where he wants it to be romantic — and she’s trying to grapple and deal with that change as well.”

Grace (Esther Keyes in “The Handmaid’s Tale”) said she “did extensive research” to prepare herself to play Jan. “I watched the documentary [‘Abducted in Plain Sight’] many times and I read both of [Jan’s] books,” she said. “I would also speak to her and I had the court documents and all of the letters between her and B and all of her diary entries from that time.

“Ms. Jan really gave all of us a lot of creative freedom in playing our characters because it’s her life and her story and her family, and at the same time the actors have to tell a story and bring some of ourselves into the characters,” she said. “I really wanted to make sure I that I was [playing] her right and going through what she went through.

“I spoke to her often and I still do. I really love her,” she said of Broberg. “Any time I had a question I would message her and she was always so open and honest with me.”