Agatha Christie’s novel planning was chaotic, with notes in multiple books, ideas which took years to be used and many a dead end.

I’m sure I recently read a quote attributed to her which suggested that she wrote until she was two thirds of the way through a story, then went back, worked out who had done it and rewrote as much as was needed to lay down more clues. This sounds just like the process I’m using in my current project- I just killed a completely different person to the planned victim, in a far louder way than intended. I think I know who did it.