My characters, to varying degrees, are living full-blown in my mind. I may make a few quick notes, but not much more than that. However, most of my writing is in the short story and novelette areas. I have not worked on a novel, and all of my books have been nonfiction. To add depth and life to your characters.
Most writers flesh out their characters to varying degrees before starting their story.
Especially in a novel, this can prevent you from needing to go back and rewrite scenes because a character did something that, earlier in the story, he would never do. Or to make sure the characters are dissimilar. There's nothing more boring than having two or more characters with similar habits, attitudes or ways of speaking. She creates a map of the area where the story is taking place, takes photographs of the area, or an area like the one she envisions, and has pages of information on each character. How they look, dress, comb their hair, their family tree, schools attended, etc.