> I wonder if they go back and add details in or plan from them way back at the start.

I heard a rumor that what Agatha Christie (and perhaps other mystery writers) would do is to write the entire story with no perpetrator in mind; then at the end, see which character seemed the least likely suspect, and then go back and "frame" that person.

On a slightly different note, when I've played the game "Once Upon a Time", which involves structured competitive / collaborative storytelling in a group, one of the hints I always give is to never specify anything unless you need to. If you don't specify what color the sword was, or what town he grew up in, or where the horse came from, then it's easy for later storytellers to incorporate that into their story. (Since although the goal of each player is to bend the story to their own ending, the purpose of the game as a whole is to tell a good story and have a good time. It's more fun to have a satisfying story someone else ended than a stilted, unsatisfying story that you ended yourself.)