Aubrey Plaza, Jean Smart, and Dan Stevens have all joined the pilot from Noah Hawley.

Aubrey Plaza is probably best known for Parks and Recreation but has also been in such nerdy things as The Legend of Korra and time travel movie Safety Not Guaranteed.

Jean Smart has recently been in Fargo. But she was also a voice in Kim Possible and an episode of my personal favorite Psych.

Dan Stevens has been on Downton Abbey, his other work is less nerdy but he has been in Night at the Museum: Secret of the Tomb where he played Sir Lancelot.

http://www.hollywoodreporter.com/live-feed/legion-dan-stevens-aubrey-plaza-861358

The Legion pilot introduces the story of David Haller, a man who may be more than human and who has struggled with mental illness since his teenage years. Diagnosed as schizophrenic, David has been in and out of psychiatric hospitals for years. But after a strange encounter with a fellow patient, he is confronted with the possibility that the voices he hears and the visions he sees might be real. In the comics, David is the son of X-Men founder Charles Xavier (aka Professor X) and Israeli Holocaust survivor Gabrielle Haller. The character first appeared in New Mutants No. 25 in March 1985.