When I was a kid, I hijacked the Art through the Ages textbook that my mother had used in college and used it to play a game akin to an inkblot-reading test. Not knowing the contexts for many of the paintings (or simply not caring about them), I'd look at the pictures and ask myself: what's going on here? Who are these people and what do they want? How do they see the world? It was hard not to look at some of them and hear how they might speak, or imagine how they might walk across a room, or speculate on how they dealt with a given problem.
The last, in time, became the most important thing. The others were easy — even a ten-year-old doesn't have trouble coming up with someone who has a distinctive appearance. Maybe too easy, because for too long in my book that sort of thing passed for the "original" part of an original character. It wasn't until I hit my late teens that the idea of a character as being synonymous with a point of view, a philosophy of things, was something I understood how to put on paper.
A person is not just a bag of attributes, in both the role-playing game sense of the term and the more generic sense. I remember an early writing-class exercise where we had to come up with characters, and one of the things the teacher pointed out was that the vast, vast majority of the things we were coming up with were either physical descriptions or just outward behaviors. So she started pressing us with tougher questions: What does this person do when someone shows up at the door collecting for a charity he doesn't approve of, and the one doing the collecting is someone he knows from town? It was hard to come up with something that wasn't just a projection of your own attitudes about things. That, in turn, demanded a little awareness of the world outside, and of other people.
The big mistake is to fall back on characters as types — in short, to use the shorthand of commercial storytelling as part of the creative process, which only leads to further impoverishment. Nobody thinks of themselves as a "type" — and if they do, they're laboring under a delusion, if you ask me. A type is an endpoint, a finished product. The raw material is the person him/herself.