You never go full retard, according to Kirk Lazarus in Tropic Thunder. Unfortunately, someone (most likely M. Night Shyamalan) forgot to give Adrien Brody the memo. Acting the actual village idiot - which isn't like some sort of ceremonial role, but genuinely someone in the village with mental impairments - Brody's performance slipped over into slapstick and just came off as a bit offensive. It's probably Shyamalan's fault for coming up with the role in the first place, but Brody really didn't have to say yes to it.