answer:How do they behave? Why do they behave in such a way. How much of their motivation are they in touch with. How do they look. What do I name them. Who are their friends. How much of their look is studied—a result of their choices in grooming or their effort, and how much is genes. It all has to work to support the plot. Some characters are easy. You’re able to slide right into their shoes, so no research is needed. Others are not so accessible. Research may be the only way to craft a believable character the likes of Dr. Hannibal Lecter, M.D. As noted in this question, no less a novelist than Mark Twain once said, “Truth is stranger than fiction, but it is because fiction is obliged to stick to possibilities. Truth isn’t.” Character development is a major part of sticking to the possibilities. Even in fantasies or science fiction, readers have to believe your characters are real people and given the vagaries of the world you put them in, they would behave as you have them behaving.