As you guessed, I am compulsive. I plan everything that is to happen in a book before I begin writing it. I keep computer files of plot questions, research questions, story notes, chronology of events. If a scene comes vividly to mine while I'm still in the planning stage of the book, I jot it down on a piece of scrap paper. The last thing I do before sitting down to write the book is to organize all the little scraps in the order I'll be using them.
Research is extremely important to a period piece.
This is useful for a writer with a day job. But research should never be the tail wagging the dog. My books are mysteries, which means that plot and character are always paramount. I love period detail, as I'm sure all historical mystery writers do, so there's always the temptation to splash it in all directions. All the same, I ruthlessly cut out any description that doesn't further my story-telling but merely shows how much research I did.