For the television adaptations of Martin's works, why exactly is it necessary to change the cast to play Targaryens? That is, a male actor for the males and a female for the females; their family tree is literally a spiral, and they would all end up being clones, inheriting the same traits from their parents (who are brother and sister), and from their grandparents (also siblings) and great-grandparents (also siblings)...

Leaving aside the fact that the family magically tanks the malformations of the rampant incest they practice, the fact is that there would come a point where all Targaryens would be exactly the same, even the women would have the same features as the men (except for their gender).