The most important thing that you as a biographer can do is to write from the heart. Write only about someone you have deep feelings for. If you care deeply about your subject, either positively or negatively, so will your readers. If you take on a biography about someone you couldn't care less about, possibly for the money, or because you have received a good publishing contract, the readers won't care about your subject either, and probably won't finish reading your book.
Brian Klems, writersdigest.com, December 9, 2013