As a player, I often (although not always) start with an archetype and then twist it a bit until it's something else.
Because of another thread, the Hermetic I played in M:tA comes to mind. I vastly prefer the Hermetic archetype for wizards, but I didn't want to be a characature. So, I started looking for something to twist. I had also just made the observation that I'd rarely, if ever, seen any of my pasty-white co-play anything besides pasty-white characters. So, I grabbed another stereotype (partially chosen because I wanted a not-nice character) of a poor, black gang-banger from Oakland, CA. The fun part was finding the intersection of the two, as one is totally anti-intellectual while the other practically oozes brains.
Another way to go is to start with a single personality trait (or two) and build the rest from there.
My screen name comes from a character who was the fullest expression of Chaotic Good I could come up with. He was pretty much a pacifist, philosopher, and had a real love of life. He was also hopelessly self-possessed and altruistic. He was so independent minded that he threw leader-types (especially paladins) into fits of apoplexy; but his moral compass, combined with an acceptance of others' paths, turned him into an anti-leader who did his own thing accompanied by scads of followers.
His brother, on the other hand, was the result of the thought, "What if a mage was so focused on creation/manufacturing/item-type magic that he'd completely lost sight of the fact that other people weren't just objects?"
Anyway, I'm not sure if that explains any actual method. All I've got, though, are examples of some of the more interesting characters I've created.