Then, when the camera starts rolling, be yourself.… The first time Lena was cast as a neurosurgeon, she panicked. She had built a modest following by appearing in low-budget online films, where directors liked her precisely because she did not seem to know the usual tricks of screen acting. She had never attended a conservatory. She had no famous parents, no Hollywood friends, and no instinct for the theatrical gestures that could make an ordinary line sound important. Her previous roles had been stranger. A nightclub worker in Detroit. A single mother who claimed to have been born on Venus. A delivery driver trapped overnight in a supermarket during an artificial-intelligence blackout. Now the script called for her to play Dr. Mara Voss, a world-famous neurosurgeon performing an awake craniotomy. She called her manager. “I’ve never operated on anybody,” Lena said. “How am I supposed to play a brilliant surgeon?” Her man...