[Growing size of the series and editorial concerns?]
No hard and fast rules here. It's all book by book. I have plenty of editors to give me input and suggestions. I don't think the success of the books impacts one way or the other. The artistic challenges would remain the same even if the books were being published by a small press for an audience of hundreds.
[Intentions behind making Cersei a POV, whether to make her sympathetic or not?]
I don't concern myself over whether my characters are "likeable" or "sympathetic." (I had my fill of that in television). My interest is in trying to make them real and human. If I can create a fully-fleshed three-dimensional character, some of my readers will like him/ her, or some won't, and that's fine with me. That's the way real people react to real people in the real world, after all. Look at the range of opinions we get on politicans and movie stars. If EVERYONE likes a certain character, or hates him, that probably means he's made of cardboard. So I will let my readers decide who they like, admire, hate, pity, sympathize with, etc. The fact that characters like Sansa, Catelyn, Jaime, and Theon provoke such a wide range of reactions suggests to me that I hav