Narrative design is the craft of designing interactive experiences in which story and gameplay are inseparable.
A narrative designer works at the system level. Where a writer creates dialogue, a narrative designer creates the structure in which dialogue functions - the branching logic, the player choices that matter, the emotional pacing that emerges from mechanics rather than cutscenes. Where a game designer creates systems, a narrative designer ensures those systems mean something - that the economy tells a story, that the combat expresses character, that the world itself communicates theme.
The discipline requires fluency in three domains simultaneously:
The narrative designer is the person on the team who holds all three of these in their head at once - and makes decisions that honor all three simultaneously.