Like most DM's, I very much enjoy world building. I feel that the secret to running players through your own world, is to always show that they have options at every point in their adventure. They can choose to save the town, or not to. However, they should always know that there are consequences for either action or inaction (just as there are in real life.) As the DM, we are there to simply create and describe the setting and characters in the world. Sometimes it's necessary to describe the physical reactions of a player (eg: Rubeus felt his knees lock, a chill shiver down his spine, as the undead vampire slowly arose from his coffin.) It adds dramatic effect to the situation, as the players may not always be expecting of a certain situation to arise.