"Try to imagine the test-chamber sequence at the beginning of Half-Life if Gordon Freeman were wisecracking all the way through, or telling his colleagues he didn't have a clue what to do. The game would grind to a halt. Instead, the player thinks, 'These scientists all act as if I know what to do, and I can't tell them I'm a complete ignoramus.' I live to create that kind of tension in the player."
Marc Laidlaw, on the mute protagonist (source)