Starting out, I'll know I want to write a particular type of novel - space opera, alternate history, crime thriller - and I'll kick around a few ideas. I'll often start with a half-formed idea. For the first Ack-Ack Macaque book, my initial idea was a murder mystery set on a city-sized airship. I wrote several plot outlines, keeping some bits and ditching others, until I had the vague shape of a story. I had the essential ingredients - the airships, the dream catcher technology, and the main character investigating the death of their ex, who was being carried around as en electronic ghost in their head.
But it was only when I realised I could slot Ack-Ack into the story that it finally came alive.
And that's how I work.
At the moment, I'm trying to write a crime thriller. I've gone through ten different plot outlines, pruning away the parts I don't like and keeping the parts I do, until I've come up with something that's hugely removed from my initial ideas, but definitely a product of them - the same way a chihuahua is a product of a wolf. It's an evolution. Each draft of the outline is better adapted than the one that went before. And adaptations that don't work are left behind in favour of new ones, until at last I've created the perfect monster... Mwhahaha!
Sorry. Getting a bit carried away there.
But hopefully you get the gist of what I'm trying to say. For me, coming up with a novel is a two-stage process. First there's the initial idea, then the refinement of that idea.
Ideas and characters accrue until the whole thing achieves a critical mass and sparks into life - and I know I have a story I can write.
Then I send the outline to my agent to see what he thinks, and he'll usually come back with some points I haven't considered. But that's great, because it helps further refine the idea. It makes sure I have the bases covered.
Sometimes, I'll write a few chapters before realising I need to change the outline again. Sometimes these chapters are filed in my archive file, never to see the light of day; and sometimes, I can cannabalise the best parts of them for later drafts.
As to where all these ideas and refinements actually originate... That's the real mystery, isn't it? I guess everything I've ever read, experienced or watched has been filed away in my head somewhere, and occasionally, unexpected connections or associations are made between previously unrelated thoughts.
Sometimes those connections are stupid. But sometimes, as when I absently jotted the words 'Ack-Ack' and 'Macaque' next to each other in my notebook, they lead to all sorts of unpredictable places.