I posted this in the comments section but I thought it wound up being so long that I should post it here where it's easier to find.
That is a hard one to just answer right off. It's a long, complicated, and ultimately changing experience. I suppose I describe it something like giving birth. I create these characters, give them life, and then they're out there. They really do exist in some hard to define way. Once they are properly living, it's only a matter of listening to them and writing down their story. I am certainly conscious of creating obstacles and story lines but in an indescribable way, I am only the conduit. Sometimes I come up with something that I really want a character to do or to say and I just can't make it work and then I realize that it won't work because it's not true. It didn't happen that way. That character never actually said that. He never would.
I do force myself to write all the time, even when I don't want to, but I rarely get anything accomplished unless I am in the right frame of mind. It' not so much a muse as a mood. Maybe they are the same thing, I can't truly say. Typically, I go through fierce sessions, lasting maybe months on end, where I hardly eat or sleep for writing. I can write a whole novel (well, the first draft anyway) in practically one, very long sitting. And then I am so drained that I don't even put pen to paper for another series of months.
And that' another thing; I can't write on a computer. I have to write with pen and paper and then type it up later. It' something about the aesthetics of it. The real feeling of my body connecting to the words. And the sound. I love the sound of pen scratching on paper, but I can barely stand the clatter of a keyboard. Although, I do like the noises of old typewriters.
I also find liquor and cigarettes to be essential to the process. I can't help but think it' no coincidence that all the greats were alcoholics. Writing is not necessarily "good for you" but it can't be helped, and in the end, writing is better than not writing. I do believe if I wasn't writing, I would probably be killing people. Franz Kafka said, "A non-writing writer is, in fact, a monster courting insanity." He has it exactly right.