The final semi-major detail I’m trying to sort out for Secrets is the backstory of the characters. As a pantser, I almost never know this stuff before I start writing. It’s only when I get partway (or a lot of the way) in and I have a sense of who the characters are and what the conflict is that I understand what the backstory needs to be. And sometimes even then I don’t know everything exactly. Usually I end up finishing the first draft and then crafting a backstory that will fit the "frontstory" I’ve written.
Ironically, the backstory is one of the most important pieces of the novel’s puzzle. It’s the foundation that all the rest of the story rests upon. All of the decisions the characters make, and often the conflict itself, are a direct result of their backstories, of everything that happened before page 1. A weak foundation can carry a story as long as it isn’t asked to bear too much. A strong foundation is even better. But a strong foundation obviously takes more work to create.
The hardest, at least for me, are the backstories for the villains. Trying to find a plausible reason that the character does the bad stuff s/he does, without resorting to cliches. I absolutely refuse to use "crazy" as a motive/reason for a villain’s actions. I also don’t really like just plain selfish/greedy. I want there to be something deeper, more of a story, some kernel that we can understand even if we can’t relate. But when the villain is willing to kill to get what they want, that becomes more challenging.
So, that’s the last big thing I need to do: sort that out and adjust the story details accordingly. I’m close. And then it’s just tweaking and tidying!