Insights of the Middle Path

Everything in moderation is the key to the Middle Path. There are times when one can clearly see the outcome (insight or out of experience) and actions of other person and their way of thinking. What is the right way to react in those situations: 1. when one doesn't have any take away from situation / outcome. 2. when one can gain from situation/outcome because the action of other can affect you or you have stake in the outcome. Does one just observe and let the nature/ karma take its course or act to moderate the outcome?