Taylor-Joy, with her delicate features and wide-set eyes, projects a quality of vulnerability and surprise. When her character teases her little sister in a cruel, frightening way, it clarifies the insidious nature of sin. The film opens with Tomasin in prayer, confessing her sins (including violating each Commandment in thought...like murder? Adultery?) and asking for the strength to be better. You believe she wants that. You also believe that she's already too smart for this shit.