CART BEFORE THE HORSE
Generally I don't delve into characters' lives offscreen. There is a school of writing that would have you work out who your characters are in some depth before writing them. I'm more story oriented; I think the needs of the story define who they are. I also think that working out the characters in the abstract is risky: you may know the characters so well offscreen that you forget to give them characteristic things to do onscreen, so your viewers don't have any clue who the character is.
But now I've got my heroine meeting up with an important character, and I have no idea how he should react. Here's where I think I need to write up who exactly he is, in order to see how he'll react. So it's not entirely a useless method.