Starting a new book is all steam and excitement, an idea pulsing with life. But I'm never sure, until I've written my way into it, whether it will really work. I have abandoned first drafts after a thousand words, two thousand, ten thousand.
Usually by the time I hit ten thousand, I have a sense of whether this is going anywhere. If the story's getting deeper and more complex, if new subplots are opening up, if the characters are revealing more with every scene, then I may have something. But if the initial impulse has burned out, its promise dwindled, the characters never growing, no new conflicts arising naturally, then it's another scrap for the scrap box. Part of it may be quilted into another story eventually.
When a story does go, when it grows legs--or better yet, wings--there's a feeling of inevitability. Yet the first shovelful into the ground (to mix metaphors here) rarely tells me how rich the pocket of ore will be. I have to dig a while to test it.