I love when my imagination takes over when I'm writing. I started working on a scene in my novel and I had this clear idea of where I wanted the scene to go but then the story took over. A character came out of nowhere and suddenly introduced a part of my main character's past and I was on auto-pilot.
I know there are many writers who find the actual writing difficult but this isn't my case. I can sit down and write for hours (if I'm ever allowed an uninterrupted moment) but it's not always this automatic process where the story takes over directing the action. When this happens I know I'm at the heart of the story and I'm not simply the puppet-master just putting things in place.
I don't have any ritual for making this happen. I don't light scented candles, connect to my muse or play soothing music. I actually find my imagination takes over in the oddest places. I've written in waiting rooms, in bars and restaurants (my favorites), early in the morning and late at night. A good run (and even a bad one) can trigger the process as well as a glass of wine and a nice soak in the tub.
The real secret, to paraphrase, Jim Lehrer, is putting butt to chair and just writing.