In The Guest, Dan Stevens plays the titular guest. As the story goes, he is a recently discharged soldier who shows up unannounced at the home of a fallen soldier he claims to have known. But then things get thriller-y! You'll have to wait until the fall, when the film premieres, to know what his secret is. But at least now you know why he left Downton Abbey: After years of being a good guy, he just wanted to play a skinny creep.