Sean Bean had a bit of the noble thing going in the first LotR movie, so I think he can do it. He just has to be even-tempered and cold, which is his normal bad guy technique.
I have only seen Addy in that sitcom he was in called Still Standing. He can do the "jolly Robert", I think. We'll see about the gruff, angry Robert. At least he never needs to play stout, strong Robert.
Serephina, I just copied the synopsis from Wiki cuz it's pretty good.
The story of A Song of Ice and Fire takes place in a fictional world, primarily upon a continent called Westeros but also on a large landmass to the east, known as Essos.[4] Most of the characters are human but as the series progresses other races are introduced, such as the cold and menacing Others from the far North and fire-breathing dragons from the East, both races thought to be extinct by the people of the story. There are three principal storylines in the series: the chronicling of a dynastic civil war for control of Westeros between several competing families; the rising threat of the Others who dwell beyond an immense wall of ice that forms Westeros' northern border; and the journey of Daenerys Targaryen, the exiled daughter of a king who was murdered in another civil war fifteen years previously (War of the Usurper) and now seeks to return to Westeros and claim her rightful throne. As the series progresses, all three storylines become intensely interwoven and dependent on each other.
The writing is really well done and the characters are all so real. I love the way certain characters have slowly changed over the course of the current 4 books. Martin's writing makes it all feel so natural.
The story is told in 3rd person from a different viewpoint per chapter. There are usually 10 or so viewpoints per book that he rotates through.