My nearly useless degree is a Bachelor of Fine Arts in Creative Writing. Yeah, I'm a certified 'artist of fiction writing.' Not that it does me any good monetarily.
I'm still plugging away at a screenplay with a writing partner, and we've got hopes that it will actually sell. We're fairly confident in it. Anyway, how this relates to gaming is in a simple trick some writers use to get a handle on who a character is and make them complex, even if they're just a minor nobody who's only in one or two scenes.
You hear movie or book critics talk about flat/2-dimensional characters and well-rounded/fleshed out/3-dimensional characters. And it's actually not that hard to create them. Basically, it all boils down to giving them a Want and a Need (some refer to these as a Conscious Desire and Subconscious Desire as well).
A Want is something your character is actively pursuing. It's the goal of the quest, the fortune and glory, the prom queen, a little peace and quiet, whatever it is they think will make them happy.
A Need is what will actually make the character happy/fulfilled.
For example, in the movie Predator, Dutch wants to get him and his men out alive. But he has a Need to prove himself the biggest badass. That's why, when he learns he can hide from the Predator with mud camouflage he doesn't use it to escape, he instead sticks around to kill the alien that wasted his crew.
In Stephen King's Dark Tower books, Roland has a Want to get to the Tower and do whatever he has to do there. But he has a Need to form a new Ka-Tet before he gets there.
Spiderman has a Want to stop the villains, and a Need to protect his family at all costs.
Of course, more complex characters can have multiple Wants and Needs, but for a beginning RPG character or an NPC, one of each should be enough. And of course you can mix things around, having a character with 2 Wants but no Need yet or vice versa. And of course Wants and Needs can and should change as the character progresses and gets what they Want/Need, or learns that they don't really Want/Need it anymore.
Of course, these really only work if they come up in play. So for an RPG, they should be thematically appropriate to the game being played. For D&D, a Want to become a world renowned florist, and a Need to come out of the closet to my parents likely won't affect play much at all. Save that for some weird furry LARP or something. But a Want to discover the lost grimoire of Yeffal the Mad and a Need to rescue my homeland from its serpentmen overlords is completely appropriate. And situations will come up in play where the player will need to choose between the two.
And that's what makes a character well rounded.