>>2048 (OP)I tend to make a list of the main characters at the beginning, just writing down all sorts of tidbits about them; physical description, likes, dislikes, family background, etc.
This is especially useful if you don't know where the story is going or how it is going to end.
Some, or most, of these bits of background/info may not even make it into the story, but I find it is useful to have established some sort of basic character before going into the story. It makes it easy to decide where that character is going, how they would react in a certain situation if you know a little about their background.
Came across these notes J. K. Rowling made when she started writing the Harry Potter books, detailing the blood status for each character was something she thought to include from the very beginning. The empty circle symbolises a pure-blood father, the filled square a non-magical father. The Star of David symbolises the mother's blood status, while the square with an 'N' inside means a non-magical/non pure-blood mother(?). Finally the letter at the end is the house they end up in at Hogwarts (Hufflepuff, Ravenclaw, Gryffindor or Slytherin).
I could not find anyone talking about the fact that Rowling used a Star of David to symbolise the mother's blood status; is she implying that the witches and wizards are like Jews? The serpent symbolism of Slytherin is also interesting, especially since these witches and wizards are die-hard blood purists.