Leonardo Dicaprio received his first Oscar nomination for playing Arnie Grape, Gilbert Grape's developmentally disabled brother in What's Eating Gilbert Grape.
Leonardo Dicaprio plays the type of role which is usually described as baity, and it is the sort of role that one can easily fall over their face in like say Sean Penn in I Am Sam. Although this is a role which can easily just be a display of someone saying "I'm acting!" this is not true though in Dicaprio's case. Yes Dicaprio is doing a lot of vocal and physical mannerisms but they never ever come off as forced or unnatural. Dicaprio makes them as an actual part of Arnie, and they help fulfill the character rather than being just some sort of acting stunt.
Dicaprio never goes for the cutesy sort of way, which some actors do when playing a mentally handicap character. He makes his manner as the true nature of Arnie, and makes them as natural as possible. He sticks to his manner the whole film through, and never loses them unnaturally nor does he ever do them forcefully. He still allows emotion from his character, but emotion that correctly works with his mannerisms and voice. He makes Arnie an interesting character and a very memorable one throughout the film. I'll admit that performance like these usually do not impress me that much, but Dicaprio really exceeds far beyond many other performers who try similar characters.