Brosnahan was still in high school when she landed her first major role in horror film The Unborn (2009). Despite the fact that she was initially told she was too young and too short for the part, Brosnahan won the role, but very nearly turned down her big break.

"I got a callback and it turned out I was going to be out of town the day the part was shooting," she later told Zimbio. "The director, David Goyer, asked me, "Would you rather be in a major feature film or take a vacation?" I was 17 and still in high school, and said, 'Well, I've had this vacation planned for a very long time.' They must have thought I was a complete moron."

Fortunately, Brosnahan decided to be in the film. "I had no idea what I was doing," she said. "I'm lucky that anybody ever gave me a job again."