1up.com have a fairly lengthly new interview with Irrational's General Manager, Ken Levine.

1UP: That is something I am really excited about too. It definitely seems like there's a "battle not monsters lest you become a monster" vibe to this. Is there any reward for trying to maintain your humanity? Do you have to kill the creepy little girls?

Ken Levine: No. No, we're not talking about specifics here but the game will support basically very emergent and dynamic choices with how you interact with those little sisters and that's sort of the core morality question of the game. But it's also a very different gameplay style depending on how you interact with them -- do you exploit them and hurt them or do you maybe even help them? We haven't talked about specifics on that but that will be a choice the player has in the game. Not like a dialogue box where you say you want to help them or hurt them but what you do moment to moment and how you interact with them moment to moment in a very dynamic gameplay oriented way. It really gives the player a lot of choice and it makes a very different gameplay experience depending on what your moral choices are.