I think that the role of the teacher during the implementation of an investigation is to keep her eye on the Big Idea. Always remember the goal of the investigation. Keep the focus on the Big Idea as you guide thinking towards the Big Idea. I found that keeping a list of higher order thinking lead questions handy keeps me focused on the mathematical processes that can be brought out by the investigation. After a lesson I reflect back on the journey and the destination, then reevaluate for the next day.