Like Moonymouse, I've never been one for living in the present. My fantasy life has always felt almost more vivid and real to me than my actual life. What happens between the covers of the books I read is at least as present to me as this thing we call real life. Some people would consider this a sign of being maladjusted. Luckily, for a novelist, it's part of the job description.
A writer of fiction has to invent people, places, conversations, events--and she has to believe in them, even though she knows they're spun of nothing but her own overactive imagination. How does a writer know a book is really taking off? When her characters start to speak to her, start to resist doing what she intends for them to do because it just doesn't fit who they are are turning out to be. Does this sound a teensy little bit like insanity? I guess maybe it does.
Being a writer means spending hours, days, months, even years, seeing not what's right in front of you but something else entirely--and trying very hard to make what's in your head come to something resembling life...not just for yourself, but for those beings you hope will turn out to be more than imaginary--the readers you're envisioning as you write.