Anne Rice started most recent book series "on a whim"

The Phoenix New Times has an article about Anne Rice, and it states that she began her current werewolf series on a whim. Rice, who is most famous for her vampire chronicles, is celebrating the pending release of The Wolves of Midwinter, the sequel to her first wolf novel called The Wolf Gift.
Both her Vampire Chronicles and her Wolf Gift series deal with the idea of remaining human despite becoming a preternatural being.

Do you start writing on a whim or do you put lots of ideas together over time before writing?
For all of you sticking with Camp NaNoWriMo NaNoWriMo you have my full support.