3. Margaery Tyrell
I loved Natalie Dormer in The Tudors as Anne Boleyn, as she brought just the right level of manipulation and entitlement to the role...along with her other assets. But Margaery Tyrell, a (seemingly) innocent girl of sixteen years, does not match Dormer's obvious level of intelligence and capacity for scheming. Margaery certainly does possess those qualities, but they're not supposed to be prevalent from the moment we meet here. As soon as we meet Margaery in the TV show, there's little doubt about her ability to play the game as well as anyone at court, and almost certainly better than Cersei. This removes a lot of the mystery surroudning her charcatre as she slowly unfurls from an innocent mystery into a master schemer, very much the heir to her grandmother Olenna Tyrell. And this is an obvious criticism, but Dormer is almost double the age her character is supposed to be, not to mention almost twice the age of Joffrey, her betrothed. Natalie Dormer's acting itself is just fine, but she's playing the mature, obviously clever Margaery rather than a girl not so much older than Sansa who may or may not be as naive.