Story Mode leans heavily toward the creative aspect of role-playing, which favors immersion in a fictional world — with memorable characters, events, and action — versus rules-based realism and optimization of stats.
Here are the main principles to keep in mind as a DM.
As the DM, your role is not to tell your story.
Instead, aim to create an interesting set of circumstances (people, places, decisions, and mood) that help make a story happen when the players interact with it.
Here are some questions to ask yourself after each session, to measure how successful it was:
The core system has hundreds of rules and sub-rules. Some of this structure is important, but following every rule — especially for circumstances that rarely happen — can take away valuable game time and stifle “in the moment” creativity.
As the DM, learn the rules as best you can, but when the game session is in progress, try to avoid opening the rulebook. Trust your gut. Make a decision that seems right for the situation and keep everyone engaged in the story.
The DM is on the players' side. Rules should be fudged in their favor — and sometimes bent or broken — if it leads to more interesting outcomes.
This still means that you can damage, poison, curse, scare, and otherwise push the PCs to their limits in order to challenge them.
But ultimately, the PCs are the heroes of the story, so try to encourage them to take creative risks that drive the story forward.
Likewise, heroes are not “punished” for making decisions that seem wrong from your perspective as a DM. There are only setbacks, which lead to a new set challenges to overcome.
In Story Mode, the only real failure is to be boring. The goal (and reward) is a new, imaginative story that you and your friends have created together.