Before development of GLaDOS had begun, Erik Wolpaw was writing the script for the video game Psychonauts, where he went around the office, finding people to provide voices to the words until they could add the final voices to the game.
Once he ran out of people, however, he began using a text-to-speech program. According to Wolpaw, people found the lines funnier than they were worth.
The creation of GLaDOS began with a discussion between the Valve team and Wolpaw on the narrative constraints they had to deal with. When they were designing the game, they found that they did not have enough time or staffing to use human characters, due to the amount of animation work and scene choreography involved.
A week later, to alleviate this problem, Wolpaw returned with sample dialogue made with a text-to-speech program, which was intended to be used as a series of messages relayed to the player in the relaxation vault, the first area of the game.
The team liked the voice, describing it as "funny" and "sinister", so Wolpaw decided to add this voice to other test chambers, all the while trying to think of story elements.
The developers noticed that play testers were more motivated by the voice because they became attached to it. As a result, the team decided to make GLaDOS the narrative voice of Portal.
