Character Study, help

      Before bed working on a older story I never finished.   Its about a mouse and a hawk helping each other...Any suggestions,or good examples of,character development or of back-story character sheets would be appreciated! After my last story, I am really trying to improve my characters prior to flushing out details of my story.   this was a great reference article. I am Looking for more like this,  or examples of how others do this differently.
How much detail do you put into characters, and how do you format or use that reference?