certainthings

It can vary, but generally, I find I have to have at least a fear, a want, and usually also a flaw (which is usually somehow connected to the fear and/or want) in mind for all the active characters in order for them to properly motivate the rest of the story. I find if I don't know those things about them, then I don't know enough about how they'll interact with others, or react to situations they're in, or will make important decisions that set things in motion. And if you don't know that, it's easy to make them do things that don't end up making sense. I always stay open to things changing, and realizing new or different things about the characters as I go on, but it's good to start from somewhere.