I'm truly curious. What is it that starts the novel in your head? Is it a character? A situation? A source of angst that makes you want to "speak" your two cents on the subject? Perhaps the plot starts to swirl and you need to fill in the details? What is it?
For me, I think it often starts with a character. This could be why my novels are so character-driven. My focus is on feelings, human interactions, and cause and effect. When I "meet" a character who intrigues me, I develop his background and record the bits and pieces of his life that he generously shares with me. Or she. (Does that sound crazy?) Granted, not all details are provided by the people in my head. Sometimes I have to embellish the details.
Now you know what starts me on an imaginary journey. What is it for you?