Wow. I've really found these posts interesting.
To me, writing a full length novel is a massive commitment of time and creative energy. I would never dream of beginning chapter one until the ideas and characters were so complete in my head that, if I didn't begin writing, my head would explode. Really. I think about plot directions and side plots and characters. I think about possible endings. I think about settings and points of view. I think a lot about voice. I think about all these things (once an idea materializes) until I literally can't sleep at night. Then and only then do I write word one. After that, the passion drives me to get it all out and I couldn't possibly think of anything else. I often stay up all night with the words flowing faster than my fingers can get them down. Stephen King calls it "falling into a hole". The world could end around me and I would not notice. There is no chance any new character or separate story line could distract me into beginning a different novel. I literally can't stop until it all spills out. In fact, this rush of words is my absolute favorite part about writing.
Should a new story idea enter my noggin, I might jot a note or two about it so I will remember it later when my WIP catharsis is complete, but I'd never let it butt in front of the line.
I can't imagine not writing with a passion that drives the process forward at this level of intensity. Really, if your story isn't riveting enough to keep you at it, how interesting do you think a random reader will find it? If my interest wanes in my characters or story as I write, something is deadly wrong. Fatally, probably.
But then everyone has their own way. For me, without that incredible rush of excitement as the story sort of writes itself, I'd be bored stiff.
