You might think twice about overlooking the office wallflower after watching Ella Purnell play a sad sack gone sadist on the new Starz dramedy “Sweetpea.”

The British actress, who is also an executive producer on the series, plays Rhiannon, a shrinking violet who develops a thirst for vengeance after her childhood bully resurfaces.

Purnell, 28, is best known for playing a high-school princess on Showtime’s teen survival drama “Yellowjackets” and a sheltered innocent on the postapocalyptic Prime Video series “Fallout.” Now she’s traded their perkiness for pent-up rage. She called Rhiannon “a case of arrested development” during a phone interview in September from Los Angeles, where she’d attended the Emmy Awards ceremony the day before.

“She’s stuck in the past and unable to move on,” Purnell said, adding that she’s the opposite in real life. “I really love processing things. You can’t just stuff things away in a little box in your brain and not think about it. It will find other ways to come out if you don’t feel your feelings.”

Still, there are some things Purnell — a self-described creature of comfort — won’t let go of.

“I like to do the same things every day. I like to eat the same things every day. I get hyper-fixations,” she said before discussing worn-in footwear, a miraculous beauty product and the American meal she craves. These are edited excerpts from the conversation.

Breville Espresso Machine

I spent a lot of money on that machine; it’s not going to waste. I used to work in a coffee shop, so I have experience tamping down and getting the right grind size and pressure. Sometimes, before I go to bed, I think about how good my coffee’s going to be in the morning.