Oh, fun thread! ^_^
Most of mine start with an OOC reason that I want to play out. The good ones become roles I can play as if it were improv theater. Stats and descriptions don't seem to make for long-term enjoyable ISRP play as much as a personality you can sink your teeth into (allusion intended, Cyath, hehe!). Thinking strongly about motivations and back-story helps with that.
Kathryn_aka_Kat was my first ISRP char. I'd scoped out the message boards first and everyone was complaining about high-level godmoders with tons of magic. Sooooo... she was human rogue-type, low level, not a single bit of magic to her. Funny thing, she started with four daggers. A year later she still had four daggers and now all were magic. People just gave them to her!
Kat tended to avoid combat in favor of diplomacy. I wanted to become more familiar with combat mechanisms and how to translate them into ISRP play. Sooooooo... Chelyrra is the anti-Kat in terms of diplomacy and willingness to fight. And she was developed at a time when there were a LOT of nicely-nice Drizzt-type drow, feh on 'em, so she isn't. She's had dozens of fights and lost all but 2 of them, because I still prefer playing low-to-mid level chars. Hasn't done a thing to reduce her arrogance though.
After exploring combat, next comes, you guessed it, spell-casting. Enter Saera. No fighting skills, not very diplomatic, but a nice girl with lots of spells. And I'd been playing a lot of Kat with a mind-blank ring where I couldn't post thought balloons. Sooooo... Saera keeps a running commentary in her head, where some of her polite "niceness" gets translated into what she really thinks. Avoids too much saccharine. The crafting proved a great way to make connections with people and gives an excuse for what she's doing with her life other than the tavern.
Having now egotistically told you all about coming up with -my- chars, I do want to make one point. Whatever char you have, it has to matter to YOU. Maybe you like it, maybe it's the sort you love to hate, but you need some interest in it and how it relates to others and how its life is going to progress. Because otherwise, you'll be one of those sitting around bored waiting for others to pull your puppet's strings for you.