Despite coming from a family of actors, Thomasin didn't initially grow up thinking she would be an actor herself. "I never wanted to make a career out of acting," she told New Zealand news site The Spinoff. "I remember my grandma asking me what I wanted to be when I grew up, and I said every single job I could think of. After doctor, zookeeper, lawyer, vet, model, I finally said, 'but I do NOT want to be an actress.'"
"It's not something I always wanted to do," she said in another interview. "I kind of just fell into it when I was 13 and became really passionate about it." She changed her mind about acting after working on Consent: The Louise Nicholas Story about a real-life New Zealand woman's experiences of sexual assault. "My whole understand of what acting is changed," Thomasin said. "I realised I could tell stories through acting and maybe make a difference somehow."