 GLaDOS returns from Portal as a major character and the game 's antagonist , and is voiced by Ellen McLain . The writers found that they needed another character to play off Johnson , but did not want to hire another voice actor . Having already recruited McLain to play GLaDOS , they asked her to provide the voice for Caroline , Cave Johnson 's assistant . Wheatley is voiced by Stephen Merchant ; early demonstrations at trade shows used the voice of Valve animator Richard Lord . The writers wrote Wheatley 's lines with Merchant in mind , citing his unique " vocal silhouette " and his ability to ad lib in a " frantic " manner . They had assumed that Merchant would be unavailable and contacted The IT Crowd 's writer Graham Linehan to try to get Richard Ayoade , but then discovered that Merchant was interested . Merchant spent around sixteen hours recording lines and was given freedom to improvise . 
