For me a character has to remind me of a real person, their actions and reactions have to be realistic. If they are angry they'll act angry, if sad, they'll scream and cry and rage. If happy the will laugh and cry - and I'll be right along with them.
I don't have character sketches or interviews or anything like that for my characters. They introduce themselves to me and I get to know them as their story unfolds.
Many times I'll ask questions of them and they'll answer but it's usually in the course of writing the story.