It was all written at once, during the pre-production phase. We had daily meetings to brainstorm settings, characters, goals, puzzles, etc. The next day Peter Chan would come in with sketches or storyboards. Meanwhile I tinkered on making the engine work. Once the doc was done (and some locations/puzzles/characters were cut out to reduce the production cost estimate) and the engine was able to display one character walking around one temporary room, we went into production, where the team size ramped up and we churned through the document building locations, modeling, texturing, and animating characters, etc. This doc became foundational for anyone joining the team to learn what the game was supposed to be like.