George R. R. Martin dropped some hints regarding the the five roles cast so far, and hard on that revealed that a sixth was cast. The hints are quite obscure, but they’ve lead fans to speculate quite thoroughly. As for the sixth role cast, we do know that Daenerys had been narrowed down to three parts recently, and it may be that they have made their choice and struck the appropriate deal with the actress.
The hints are as follows:
“The five roles… well, we have cast Jaime Lannister and Theon Greyjoy, I let that slip in Montreal… the three new ones are all females. A couple small roles, but very important ones. One player has something in common with Ned. One has something in common with Harry Potter. One is not a boney pop singer. Shows can get cancelled and so can characters, but two of them have been seen on American tv recently.”