When I started writing that book, I had a setting (a fictional town in Pennsylvania based upon an actual area where I had lived), some loosely defined characters and the prologue (which I had written down after a dream). I had no idea who the victim was, when she was killed, how or who killed her.
This is when I discovered that I love writing "by the seat of my pants". For anyone who knows me, this is counter-intuitive. I am a detailed analytical. I love to plan. I am SO not impulsive. But for whatever reason, writing without a strict outline works for me.
I used the same method with book two in the Klondike mystery series, "Barely a Spark" and will be doing the same this year with book three, "Almost a Touch".