The cast also includes Samantha Coughlan, Ryan Windlsey and Christos Lawton, previously appeared in one episode of Downton Abbey.
Fletcher is best known for his role in ITV soap Emmerdale, in which he played Andy Sugden for over 20 years.
The actor departed the Dales last year and recently ruled out the possibility of returning.
“Emmerdale was 20 years of my life,” he said during an appearance on Lorraine. “It was a huge part of my professional life and my personal life.