When she landed the part, Berthelsen hadn’t yet seen Stranger Things, but she went on to binge season one like the rest of us. While watching, she searched for clues about her character, since not much was revealed to her by producers, but since her character forms the central part of a standalone season 2 episode, there wasn’t much to find out.
“I got these scenes that were so different from the first season. They had nothing to do with the plot from season one. While I was trying to find out what I was going to be involved in, I was really just excited about the rest of the characters. But I was searching for clues about my part.”
“[My audition material] was only a breakdown and three scenes. It didn’t have any of the characters’ names on it. It only had four lines about her: “She’s emotionally damaged, she has a hard time connecting with other people, she suffered a great loss as a child, and she’s been seeking revenge ever since.” That’s all I knew. I had to create the character based on this breakdown.”
(via)