I like picking the major themes of my character and then letting mannerisms and catch phrases come as the game goes on.
Example, today was DnD Gameday for the DMG2, I hadn't ever played a fighter type character but Tom and Mike have convinced me to at least try it out- especially Tide of iron.
So i chose Xadross 6thlv human fighter Iron Vanguard type- worshiper of the Raven Queen and unaligned, he has sweet shield combo powers. So i decided that he might just try to use his shield all the time, not just in combat. Eating off of it instead of a plate, sleeping under it. And since he is a meat shield (defender), just getting in the way of people trying to kill others, ALL OTHERS.
Anyhow, during the first encounter I could have just hacked away at a lone Duregar but instead lured him to the edge of a cliff with my At wills and then bucked him off with my shield.
Then during the second encounter a Duregar was about to have his limbs cutoff by a Doomdreamer, so i jumped over the river, and used an action-point to free the duregar and flip the Doomdreamer into the river.
Another tactic for roleplaying that I use is a fetish. I fixate upon an object of some significance, like if I'm a druid I fixate on oranges, or if I'm a paladin I focus on communion wafers. Its pretty annoying but having just one idea and branching out from there helps (me) a lot.