When I wrote my first novel, I still had quite a bit of learning to do. I consider the entire process one big educational experience. When I wrote my first draft, I didn't feel like I knew my characters very well. As a result, some scenes were difficult to get right. Reactions and dialogue didn't seem genuine.
For the second draft, I decided to go through the process of fleshing out the characters and as one would imagine, had a much easier time writing after that. The experience taught me that writing without fleshing out one's characters is like marrying a stranger. You just don't know what you're getting yourself into.
For my current writing project, I decided to start on the right foot. Not only did I want to get to know my characters, but I knew that I needed to keep this information organized.
Main Characters: I did some research online and found several character worksheets. They usually vary a little bit, but I took from them what I needed. Most character worksheets include things like first childhood memory, character's biggest secret, pet peeves, etc. A lot of this won't go into the book, but it helps you to get to know the characters and create better dialogue, reactions, etc. To stay organized, I keep a worksheet for each separate character saved on my computer in a file folder.
I consider my "main" characters any individual who experiences growth throughout the story. Obviously there will be certain characters that pop in and out and don't necessarily need the same treatment. That brings me to...
Secondary Characters: For these smaller roles, I personally don't feel that a worksheet is really necessary, though you do need to make some decisions. At the very least, you need to know last names, ethnicity, hair and eye color, and a basic cultural/education/financial background.
Once I've figured out all the details for each character, I write the most important ones on flashcards. Sometimes when I'm writing I can easily forget certain details. The flash cards are a quick, easy reminder when I need them.
Now that I've got the characters under my belt, I'm well on my way to my first draft. I'll keep you all updated with my progress. So far so good!