Know thyself. As a person and as a writer. It makes for better characters, better dialogue, better stories, a better you.
I am on Holly Lisle's weekly email list and I've never been disappointed with the topics. The emails themselves are simple in nature, the goal being to teach you how to make you a better writer. Each is a few paragraphs, usually with several resource links, and the subjects are always thought provoking.
She recently did a multi-part string of emails that made you step back and really consider "you". Your flaws, fears, hopes, dreams, scars, lessons learned, etc. It's not just about that introspection that we all need sometimes, the focus is to get you in tune with those things and drive you to put them on the page. You don't have to be writing an autobiography to inject those deeply personal touches, you can work them into characters and conversations. Maybe you never learned to swim, so your character can't. Maybe you almost drowned and so your character has a paralyzing fear of deep water. Spiders, snakes, fire...q-tips. Who knows?
Your character be could fearful of anything for nearly any reason. Even worse, what if they're irrationally afraid of everything? How do they deal with their bizarre compulsions? Can they? Perhaps they must be medicated everyday, the degree to which they are doped is entirely up to you. Do they take a simple pill each morning? Is it an injection that must be administered by someone else? What if part of their phobia is needles, but there is no other way to get the dose? These are all things to consider. They're a normal, every day person with the help of the meds......but then what if they lose that oh so important little bottle?
You can really run with it. Maybe they have just lost their job and don't have the money for the prescription. They changed jobs and their new insurance won't take effect for 90 days. Perhaps the new insurance doesn't have drug coverage at all, or doesn't pay for their usual rx and now they have to change medications. Or they have moved to a completely new area and need to find a new doctor to prescribe. The search is proving difficult and the increased stress of the move and a new home and job are taking their toll. New symptoms begin to manifest. They are becoming more and more delusional, suffering from intense anxiety, hallucinating. I could go on and on, but I think that's enough.