I'm writing my fourth (and fifth, on pause) currently, and I am developing a method of preparing the narrative and characters well in advance of writing the book. The first two books took shape largely as they were being written, rough outlines becoming more solid as the book progressed. E.g., if I started writing chapter three, I may take a moment to look ahead at what needed to happen in that chapter, alter things that had been made unworkable by the previous chapters, solidify important events and ideas, etc.
For books three and four (currently), I had begun outlining months in advance of writing. Indeed, about 50% of my upcoming projects were all developed in a few months about two and a half years ago. Of course these ideas changed over time, sometimes so significantly as to be barely recognizable, but the impetus, the kernel, remains.
However, the one that changed THE MOST, and indeed had the longest gap between idea/outline and novel, is my current book that had very little preparation. I did a few simple outlines to gauge the plot trajectory, to map a few character arcs that I knew were important, but mostly I just let the thing ferment for a solid two years. I'm glad that I did. I've improved enough over my last three books and short projects that I feel I wouldn't have done the thing justice with a detailed outline and sharp pre-plotting. Too much would have solidified. It wouldn't have transformed into something (I think) really great. It would have been numbingly average (like my last book!).