To act 100% retarded with extra no redeeming special quality.
Downey: "Everybody knows you never do a full retard."
Stiller: "What do you mean?"
Downey: "Check it out. Dustin Hoffman, 'Rain Man,' look retarded, act retarded, not retarded. Count toothpicks to your cards. Autistic, sure. Not retarded. You know Tom Hanks, 'Forrest Gump.' Slow, yes. Retarded, maybe. Braces on his legs. But he charmed the pants off Nixon and won a ping-pong competition. That ain't retarded. You went full retard, man. Never go full retard."