As mentioned, Variety listed Ben Mendelsohn in the starring role of The Outsider, but did not specifically announce which role he will be playing.
Mendelsohn could also be playing the book’s accused murderer, Terry Maitland.
Police detective Ralph Anderson would be a possibility, as he leads the investigation and discovers the weirdness happening in the case that prompts him to dig deeper and get in over his head with a “supernatural force” that has some bad intentions.