When I started to write "Not a Whisper", all I had was a dead body, a location and a few loosely defined characters. The prologue came from a dream I wrote down in the middle of one night. That's where I got my dead body. I had no idea who the killer was and, in fact, changed my mind several times as I was writing.
Book two in the series, "Barely a Spark", killed off a character from book one. I knew I wanted him dead but again, when I started I had no idea how he died (suicide, accident, murder). And once again, the killer changed several times before I settled.
I love "whodunit" mysteries, if you do too, hope you will give my Klondike Mystery series a try!