Winstead's party-loving character embarks on a path of sobriety in the film, and she prepared for the job by spending months attending real group sessions.
However, she insisted on being discreet and never told anyone about her job as an actress.
She tells British newspaper The Guardian, "I came into it very quietly: I didn't say I was an actor. If they asked me to speak, I'd tell them I had come to learn about AA. I never pretended to be anything I wasn't. But I also didn't want to stand up and say: 'Hey guys - I'm making a movie! Anyone wanna tell me their story?'"
Winstead embraced the role in the movie, which also stars Breaking Bad actor Aaron Paul, and adds, "I really didn't have any fear about the humiliation factor. That was something I was excited to get the opportunity to explore. My fears were about pulling it off."