I've talked before about how I don't plot things a head of time. I know a few rough things I have an idea about but mostly I only know what's happening a few pages before I draw it into the comic.
Generally the way it works is when I am trying to think about what to put on the page something jumps in my mind but won't work without some kind of build up so I have to set it up for later.
Or I'll finish a page and realize the implications later. I thought of this Monkey Paw thread beat maybe two weeks ago