The next time you're researching, take some time to explore this unconventional resource: people who know nothing about your subject. In among visits to the library, Googling until you have cartoon eyes, interviewing experts, and visiting relevant Web sites, just chat with anyone willing to talk about the subject.
If you're writing a biography of Amelia Earhart, for example, mention it to your neighbor. It's likely that she will have something to say about Earhart. It might be as simple as, "I wonder if she was really lost?" or "I wonder if she married Putnam because he would finance her career?" Get enough such responses from the uninformed and you'll begin to build a profile of your reader in dimensions you may not have explored. And some people will surprise you by knowing something about your subject. When I was working on the Crosley biography, I mentioned it to quite a few people who were able to tell me things that I wouldn't have known otherwise (like the fact that Lewis Crosley had a mistress). Try it: I think you'll find that it opens new doors.