Tatiana Maslany for The New York Times

Wavy-­haired and theatrically dirty, Maslany spoke in Sarah’s lower-class British accent between takes. (She kept it up until they broke for lunch.) She was warm and self-­assured and modest and frank. She exuded a contagious ease. In our very first conversation, we bonded over the unsung virtues of the adult onesie. “I had one that had the butt-­flap until after high school,” she told me.