Crook (The Office, Pirates of the Caribbean) would play Vargo Hoat, nicknamed 'The Goat', according to Entertainment Weekly.
A sadistic mercenary, Hoat frequently tortures his victims and speaks with a lisp due to his swollen tongue.
Waking the Dead actress Fitzgerald has reportedly been cast as Selyse Florent, the wife of Stannis Baratheon (Stephen Dillane).
In the original novels, Florent worships the god R'hllor and is a supporter of Melisandre (Carice van Houten).
Sherlock Holmes actor Clive Russell will play Catelyn Stark's uncle Brynden 'The Blackfish' Tully in Game of Thrones season three, while child actress Kerry Ingram has been cast as Shireen Baratheon.
Game of Thrones will return to HBO in the US and Sky Atlantic in the UK in 2013.
> 'Game of Thrones' season three: New character list revealed
Watch a recap of the Game of Thrones season two finale below: