Tim Plester, the English actor/playwright, has been cast as the Black Walder on the game of thrones’ season 3. No official announcement/confirmation has been made but his role as the Black Walder is credited as one of his many roles on his website.
In A Song of Ice and Fire, Black Walder is the great-grandson of Lord Walder Frey. He is called Black Walder because of his temper and terrifying demeanor, he intimidates everyone including his family.
Source: Winteriscoming