Some really great points are being made! So for the sake of not repeating what's already being said, my advice would be to type up a character sheet no matter who/what you're playing. It's good to have something like that handy, even if you're in an RP that doesn't require a sheet, because you...
This is constantly up in the air for me. I.e. I was really fascinated by having a character named "Dogface" and I designed him around the idea of a junkyard dog personified: he's all skinny, scratched up, and permanently scowling. Dogface's backstory and personality came waaaay after I was...