First Pillar of Dungeon Narrator Authority
Rulings over rules. When the books don't cover it, you decide. Fast. Confidently. You can always refine it later. Picture this: Your player wants to use their rope and grappling hook to swing across a chasm while the goblin archers fire at them. There's no "cinematic rope swing" table in the books. So you make the call. Maybe it's an Dexterity Chance Roll, TN 15 (TN: Target Number is a value of difficulty that a player rolls 1d20 + ability bonus to roll equal to or higher for success). Done. Game continues. The moment doesn't break while you hunt for the "correct" answer because there isn't one. There's only your answer, and that's enough.
Second
Consistency over creativity. A brilliant ruling that contradicts your previous call breaks immersion harder than a boring but reliable one. Think of it like physics in your world. Gravity doesn't change based on what would be cooler in the moment. Neither should your core mechanics. Your players are building mental models of what's possible. Don't pull the rug out from under them.
Third
Consequence over spectacle. Players remember how their choices mattered, not how many dice they rolled. When the fighter uses their shield bash to knock the cultist into the summoning circle, disrupting the ritual, that's memorable. Not because you let them roll a big damage die, but because their tactical thinking changed the entire encounter. You're not here to dazzle them with effects. You're here to make their decisions count.