What I did:
Narrative Document
My goal for the narrative document was to create a central source of information regarding the story that the team could refer to during development. After several meetings early on to narrow down the basic premise, I began to draft the narrative document and would notify the team every time there were significant updates.
Along with details about the story, characters, world, and themes, I included several references to other media that I was drawing from to give my teammates a better idea of the tone and style I had in mind when writing.
Script
My next step was drafting scripts for our dialogue sequences. I aimed to create a narrative that had depth while not getting in the way of gameplay. I chose to have characters talk during gameplay instead of halting the player to deliver exposition outside of a couple instances.
This way, players who were interested in the story would receive it efficiently and players who only cared about the gameplay could ignore it. I used screenplay format to make it more familiar for actors to read from instead of from a spreadsheet since we knew we wanted voiced dialogue.