I recently picked up a book on acting and directing actors and found some tips you may find useful for developing characters. One of the main ideas is to try to think of yourself and how you are similar to the character. With that in mind as you fill out a character sheet, answer the questions in regard to yourself first. This may help you think of the character in a deeper way. Then ask the tough questions.
What unpleasant truths is the character forced to deal with?
What is the character most knowledgeable about?
How does he/she use their intelligence?
Is the character protecting himself from past pains or avoiding situations because of his past?
What makes him/her laugh?
What makes him/her loose their sense of humor?
What is their blind spot?
What are they doing in this scene that they have never done before?
In what way is this character an artist?
These are the questions I found were most often left out of character sheets that really help me understand the character more. By answering the questions of myself I am more able to relate to my characters and write them more honestly. They seem more human to me when I put little pieces of myself in there with them.