It doesn't happen until I start the real writing. But it's like something pent up has been let loose, I can feel the opening inside of me, and there is a strong and steady flow that can cut through rock and earth that bursts forth and begins to go steady and sure. It carries me with it to places and scenes and characters I did not always anticipate or plan for and it is irresistible and unstoppable. It can be like a fire too and hurt & burn if I do not let it out and hurt & burn even if I do. I am swept away with it until we empty into the great sea of the ending. This very much happened with Wings and Heaven.
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I do bare bones plotting and envisage what needs to happen with each chapter (as well as one can knowing the plot will take twists and turns you didn't plan on) - but I most certainly don't nail everything down tight before starting - however I have a strong sense of direction and beginning and that propels me out of the blocks - then it's a matter of thinking and evaluating day by day as you write and things take on shape - I would never presume to nail everything down before writing began because the act of writing changes everything since it brings the ideas to life & anything can happen once you have life force.