I'm not a massive fan of character bibles. You know how the book you haven't written yet is perfect and wonderful? And then you get cracking and it gets worse and worse and more and more hopeless until there's nothing for it but to finish it and move on to the next one that's still perfect(ly unwritten)?
Basically every decision you make while you write is slamming closed a door on all the possibilities you didn't choose and so for me a character bible would mean starting the book with quite lot of doors already slammed. Where's the fun in that?
There are still some things about Dandy Gilver I don't know after ten books. In fact, more than not knowing, there are things I'm determined not to find out, because I want to be able to decide when the right moment comes.
And that brings me to my Donald Rumsfeld system of what you do and don't need to know about characters.
I never understood why people gave him a hard time about the (un)known (un)knowns. It struck me as perfectly sensible. Here's how it works for character development.
1. Known knowns. You need to decide pretty early on what your character's name, age, race, gender etc are (unless . . . see 2)
2. Known unknowns. The things you know you don't know and so you don't talk about them. Perhaps very deliberately. For example, the protagonist of Rebecca is nameless. And perhaps because you want to leave breathing room for later developments. Like me with Dandy Gilver.
3. Unknown knowns. These are the things about your character that you get without being able to define. For instance, Shakespeare wrote the gloriously neurotic character Hamlet a hundred and fifty years before neurosis was named. I think you need access to the output of a character but you don't necessarily need to be able to pin it down.
4. Unknown unknowns. These are bad. These are really not good. The technical term for them is . . . mistakes. The things you don't know you don't know about your character will make them do and say things they'd never do or say.
The most common example of an unknown unknown is when you try to write a character who comes from somewhere you don't. Unless you get it checked out, you're on a shoogly peg. (A shoogly peg is Scottish for thin ice.) You're fine as long as the book is read by other people who don't know but Blimey O'Reilly it clangs when someone who does know gets eyes on it. (Blimey O'Reilly is British for Boy Howdy.)
This is right at the front of my mind just now because I've written a Japanese character (in Scotland, speaking English) in COME TO HARM and although I bugged Japanese friends endlessly, asking questions, I'm waiting to hear what I got wrong. So that's known unknown unknowns, I suppose. Suspected ones, anyway.