Fark linked to this video of David Tennant as The Doctor. In one episode ("Family of Blood"), the Doctor must hide from aliens pursuing him, so he converts himself into a human. He leaves instructions for his companion Martha, and in the show we see her listening to them for just a moment, and the volume goes down. We see him talking, but don't know what he's saying.
Now we do. This is good stuff. I can't imagine having to do this in front of a camera. It would be very tough.
Gotta admit: I though he said "Pez" until I realized it was "pears".