Exclusive: This fast-rising actor who got killed off on 'Underground' is moving to PBS's 'Mercy Street'

"Underground" actor William Mark McCullough is moving to "Mercy Street," Business Insider has exclusively learned.

PBS confirmed to BI that McCullough has been cast in a recurring role on the second season of its critically acclaimed medical drama set during the Civil War.

McCullough will be playing Larkin, who is a cohort of James Green Jr. aka Jimmy (Brad Koed Jr.). According to our source, Larkin has a violent streak and could be covering up a secret about his real identity.

The actor joins recently announced new "Mercy Street" cast members Brían F. O'Byrne ("Mildred Pierce," "Aquarius"), Patina Miller ("Madam Secretary," The Hunger Games: Mockingjay"), and Bryce Pinkham ("A Gentleman's Guide to Love and Murder," "The Heidi Chronicles").

Starring Mary Elizabeth Winstead, Josh Radnor, and Gary Cole, "Mercy Street" is currently in production in Richmond and Petersburg, Virginia. It's set to return in 2017.

Fans of WGN's "Underground" watched McCullough's character, Theo, go down in a bloody scene on last week's episode. In addition to "Underground," McCoullough is set to appear in the upcoming film "Mena," starring Domhnall Gleeson and Tom Cruise.