The comment thread to GRRM’s post announcing the completion of the first draft has proved fruitful. He’s confirmed that so far as he knows, the character of Ros, “the Red-Headed Whore”—played by Esmé Bianco—is the only one added to the series so far who is not original to the novels, and reveals that she may appear in later episodes; having liked Ms. Bianco when he met her, he even floats the idea of including her character in the novels. Martin also makes this priceless remark, in regards to the script which he called “too long and too expensive”: “Too many characters, too many settings, too many SFX shots. What meathead wrote this novel, anyway?”
Later, however, Martin speaks seriously on the difficulty of adapting the series. He emphasizes that the first season is very faithful to the novel, but he anticipates increasing difficulty as the story spreads over a wider geographic area, with more and more characters introduced, some who may become prominent in later novels but who could well be cut when their role appears small. He can’t guarantee that the future seasons will be as faithful, but “the intent is there”.