Ok, as for pilot characters, Sean has made the list for two of the characters: Eddard Stark and Jorah Marmont. I'm in favor of Jorah Marmont mainly because he's more of a man-at-arms, which Sean excells at and because he doesn't get killed off early as Eddard Stark does, but he's not as important as Eddard Stark. However, the character of Eddard Stark physically reads like Sean.
Now, if they fail to cast Sean for either of those characters, in the following books, he makes the list for Blackfish Tully, Davos Seaworth, and Benjen Stark. He's a bit old for Benjen, but not drastically so, nothing a little hair dye wouldn't rememdy. The other two characters are lesser in importance. So the Benjen Stark character is the one I'm hoping for.
The ideal situation would be for him to play Eddard Stark and with a little Hollywood magic the character gets to live on.