Lena 'Cersei Lannister' Headey and Dominic West in 300.
West was unfamiliar with the series when he was offered the role during casting for the third season. Unwilling to spend 'six months' away from his family to film in Iceland, he turned down the role and was immediately berated for it by his nephew for not being in "the only great show on". West notes that it was a great part and already partially regrets turning it down, but not the time spent away from home.
Based on West's description of the role, which was substantial and part of the storyline beyond the Wall (which are filmed in Iceland, although only in blocks of six weeks per season), he was almost certainly up for the role of Mance Rayder, the King-beyond-the-Wall and the leader of the wildlings, frequently mentioned in the first two seasons. As of this writing, the final casting for this role has not yet been formally announced (and may not yet be complete).
Sad news, as West would have been excellent in the role of Mance Rayder. Hopefully another role could be found for him that involves less travel in later seasons (cough Oberyn Martell in Season 4 cough).
West co-starred with Game of Thrones star Lena Headey (Cersei Lannister) in the film 300 and alongside Oona Chaplin (Talisa Maegyr) in the newsroom TV drama The Hour.