Technically, he's channeling a functionally illiterate here, not a mental simpleton. Nonetheless, De Niro plays it typically broad, dipping deep into his Master Thespian handbook (the downcast gaze, the pursed-lip pout) and rendering his character thoroughly schizophrenic. Separately: Being taught how to read by Jane Fonda is the cinematic equivalent of being taught how to aerobicize by Ayn Rand.
