Henderson's Tenants started out as a piece of "periscope writing": I had an idea for a novel about nanotech and wanted to see what Henderson's world might look like. (The title came straight from my subconscious, where my own tenants live what I suspect is a life of discreet debauchery that's much more fun than mine.)
So I had no outline, no synopsis. It was just "Guy develops nanobots that not only cure his cancer and other people's ailments, but transform them into something beyond superhuman...in the process getting rid of an obnoxious and oppressive government."
Now I'm just into the second 20,000 words of what will probably run to 80-90,000 words. Mike Henderson has overcome the first series of obstacles in his way, creating the code for his nanobots and getting hints of the tenants in his skull who are the code's real authors. He's about to build the nanobots (actually they get built on a printer) and to try them out on himself and his dying neighbour John.
By now, even without an outline, I can see where the story goes: two or three chapters about the immediate effects of being occupied by nanobots that replicate into the billions and grow themselves into his nervous system; then a series of major struggles: first, Mike's conscious mind has to regain some kind of control over the bots and their subconscious allies; the bots' experiments bring Mike close to death (and the same with John and Patty); Mike's Korean backers may be scared by the immediate effects; and the Homeland Security people, who've been watching from a distance, will decide to swoop in and intern everyone.
That should take me to 40-50,000 words. The rest of the novel will be the counterthrust of Mike and his people, all nanotech-transformed, against a society that regards them as highly contagious Walking Dead. The attempts to destroy them will fail, but they've got to come close to succeeding--even against the constantly increasing intellectual power of the human/bot combination.
I would still hesitate, though, to work out the scenes in any detail. This is going to be a case where my own tenants will have to tell me what happens next, and often that will be with no notice at all--Mike or John may walk into a room and the tenants will have to tell me what they find there, and what they should do about it.
They say Elmore Leonard writes all his novels like this, and keeps writing just to see what happens next. I envy him. It makes me nervous.