Blake Lively's upcoming film The Rhythm Section might be her grittiest role yet. The actress undergoes a dramatic transformation for the thriller, but just who is her character Stephanie Patrick? In this exclusive clip, Lively and her costars, Jude Law and Sterling K. Brown, try to explain just that.

After learning that the plane crash that killed her entire family wasn't an accident, Stephanie becomes hell-bent on seeking revenge. With a little help from Law's character, she transforms into a ruthless assassin and master of disguise. "She really is just a normal human being who was thrown into this world," Lively says in the clip. "She carries a lot of guilt for the things she's had to do." Yep, this is Lively like you've never seen her before. Learn more about her character in the video above before the film hits theaters on Jan. 31.