Valve's Chet Faliszek told DigitalSpy, "Really it was more about that they were the voices in our heads for the characters. Stephen Merchant in particular, we just had that voice in our head, and his pacing has a distinct way of speaking (and) a way of making us laugh."
"So we were happy to get him, totally surprised we could, and he was a natural fit," Faliszek added, "It was great working with an actual comedic actor himself, because he knows timing, he knows how to tell a joke and knows about pacing, and that's really important."
Portal 2 will be released next week on April 19 for PC, Mac OS, Xbox 360 and PlayStation 3.