Writers have been adding unrelated dialog to somebody else’s movies to create something satirical, fresh and new for just about as long as there’ve been movies. (Woody Allen even did a full-length feature film that way.) The fine folks over at Rooster Teeth Productions have made a virtual career of this, with a series of videos based on video taken from Microsoft’s Halo games for the Xbox 360.
The backstory article on Wikipedia is worth checking out, if only because it makes the thing even funnier than it is at first glance. Enjoy.