I attended the More Reinvention Convention in DC and one of the keynote speakers, Anna Quindlen, wrote both fiction and non-fiction. She was discussing how difficult it was to continuously cross between the two because when you write fiction, you have to "live" the story in other words - it has to become a part of you, the characters the town...etc...
I was wondering for those that write fiction, how true that is for you. How deeply do you have to become engrossed in your story writing to write fiction?