I had a nice breakthrough awhile back re: someone who was quite a 2-D, cardboard guy. When I forced myself to ask more questions about his background it ended up really changing him (including his age, profession, personality, how he interacted with the other people). It made him so much more vivid to me.

I think that my interest in episodic TV can force me to think about traits and background that could be important in a future version of my story, even if I'm not writing about it now.

Also sometimes I write out a free- association essay written in the first person by each of my people.

Another one of my recent tricks is to always think of/ refer to them as people, not "characters.