Stevens will play David Haller, a haunted man who was diagnosed as schizophrenic growing up but, after meeting the girl of his dreams, begins to think the voices and visions in his head are real. In the Marvel comics, David is the mutant son of Professor Charles Xavier. The role marks Stevens' first television gig since Downton Abbey.
Parks and Recreation star Plaza will play Lenny, David's friend who is an eternal optimist, despite a lifetime of drug and alcohol abuse. Fargo's Smart will reunite with Hawley to play Melanie, an unconventional therapist. The trio join previously announced star and fellow Fargo alum Rachel Keller, who will play Syd, a scrappy woman who puts on a tough front but still believes in happily ever after.
Hawley wrote the Legion pilot and will serve as executive producer alongside X-Men: Days of Future Past's Bryan Singer, Lauren Shuler and Simon Kinberg. Head of Marvel TV Jeph Loeb, Jessica Jones' Jim Chory and Fargo's John Cameron will also executive-produce.