This is a template for the questionnaire I use when I’m at a stage of revising a project where I already know my characters and narrative fairly well but need to get to the next level. It shouldn’t be used for developing new characters, because it leaves off a lot of things you’ll need to get to know a new character and should know before starting this. Their voice, their attitude, their past, how they act in different scenarios.
This questionnaire is designed to help you get a fully formed sense of your character as being more than someone who serves your narrative, as someone who was living a life and was a real and complicated human with goals and hobbies even before the story began, and is a combination of analysis and biography. I use it for screenwriting, but it should work for other forms of fiction as well.