The show stars J.K. Simmons as Howard Silk, "a lowly cog in a bureaucratic agency, who discovers that the agency he works for is really guarding a crossing into a parallel dimension," a press release explained.
The West Wing and Ballers alum Schiff will portray "Director of Diplomacy Roland Fancher, an expert litigator and feared power negotiator between both dimensions in the series," while The Crying Game and Interview with the Vampire veteran Rea will play Alexander Pope, "an éminence grise who pulls strings from behind the scenes and knows where all the bodies are buried."
Bolger, who is from Once Upon a Time and The Tudors, will play "Anna, a mysterious young woman whose appearance on the 'other side' threatens to disrupt the tenuous truce between Howard and his counterpart."
The ensemble also includes the previously announced Olivia Williams and Harry Lloyd.
Starz ordered two seasons of 10 episodes each of Counterpart in 2015. The series began production in December in Los Angeles and will continue in additional locations across the United States and Europe.