The HBO series “Game of Thrones“, based on the “A Song of Ice and Fire” series of books by George R.R. Martin, has begun casting actors and registering extras for the third season. The Medieval fantasy drama serial will not be holding any open casting calls for extras but are accepting registration from new talent through online submissions. Principal actors are being cast in London, while the background actors are hired in Northern Ireland. Some smaller speaking roles are also cast in Ireland.