I have a general idea about what my stories are going to be about, but mostly I just know my characters and drop them into different situations. People react in a variety of ways based on their personalities, values, life experiences, etc. It's the same with characters. The best thing about being a writer, for me at least, is when I'm writing a scene, and something even I didn't expect happens. Sometimes I think, "Whoa! Where did that come from?" Little things like that make me love my job the most.