A casting call put out by Valve provides significant hints about the story and timing of Portal 2.
The company's request for a voice actor, apparently placed on the subscriber-only site Breakdown Express, provides a detailed description of the character's background, accent, and personality transition in the game.
The role is that of Aperture Science Labs' dead CEO, Cave Johnson, who, according to the brief, becomes the "principle protagonist" as the story progresses and speaks with a slight "Southern/Western" accent.
Johnson apparently goes from happy extrovert to isolated maniac, perhaps becoming a prototype for GlaDos, the mocking and acerbic narrator who taunts and teases the player in the first Portal (pictured).
The casting call indicates voice work for the character will begin toward the end of July.