"Before the writing started, we met with Erik and discussed our list of narrative constraints. Since at the time we were using some Half-Life art assets, and because we wanted to leave ourselves the option of someday using the portal gun in a Half-Life game, we decided that the story should in some way connect to the Half-Life universe.Exclusive: Inside The Making Of Portal
Practically speaking, we didn't have sufficient time or staffing to add any human characters, which would have required an impressive amount of animation work and scene choreography. That meant the story had to be expressed without the benefit of any visible extra characters.
A week after the meeting, Erik came back with some sample dialog he'd recorded using a text-to-speech program. It was a series of announcements that played over the newly-christened "relaxation vault" that appears in Portal's first room.
Everyone on the team liked the funny, sinister tone of the writing, and so Erik continued to write and record announcements for other chambers, while still searching for the story proper. At some point, however, it became apparent that these announcements were providing playtesters with the incentive to keep playing that we'd been looking for all along.
Better yet, in the sterile, empty test chamber environment, players were actually becoming attached to the alternately soothing and menacing computer guide. We'd found the narrative voice of Portal."