I am going to write my bible first - thoroughly detailing my characters, their backgrounds, the environment they move in, and how they are inter-connected in the web of their world. Then I am going to outline my plot, and allow the writing to flow!
One thing that I do know - I tend to edit to a small extent as I write. This is not something that holds me back, it is something that allows me to have a nice, clean manuscript in front of my face. Spelling errors checked, punctuation checked - life looking good!
Another important thing to decide before beginning to write is what voice do you want your book in? Who is going to be telling the story, and how are they going to tell it? First person? Third person? Do whatever you are comfortable with. Go back and reread your favorite authors. What voice do they write in? You are not trying to be a mini-me, you just want to sound real, and consistent.
My characters are already part of me - they are pushing my current project so they can get their chops out there! I know they will interact in a manner that is unique to them. I have often found that my writing takes on its own edge - it writes itself. I will have to reign in my characters, rather than trying to flesh them out! They will be the best of the best of all of the people that have moved through my life, or that I have observed along the way. My characters will "have" character, and they will "be" characters!
Some stories never get written because the writers who imagine them never move them from the imaginal world into the real world. Spend time working at your craft, people! Sit down and write! Every day!
Have fun with this! I know that I am going to!