Darlene Cates, who had no previous acting experience and was only cast because no professional actress was big enough, displays her natural talent for performing when Bonnie confronts the Sheriff at the police station.
Leonardo DiCaprio as Arnie. It takes a certain kind of knack to believably depict a developmentally disabled person, and his performance shows it in all of its discomforting glory.
The Woobie: Practically the whole Grape family. It's implied that they were all pretty messed up to begin with, but it was the senior Mr. Grape's suicide that sent them all over the edge.
Bonnie. She's eaten herself into morbid obesity at the expense of her good looks and is now the laughing stock of the town, to the point that her family is forced to burn their house with her inside it just so she won't be further humiliated by trying to get her immovably heavy corpse out.
Gilbert himself is more of an Iron Woobie: He goes through a lot out of his love for his family, even though it's shown that he has his limits.
Arnie, to a certain extent. His condition makes him a burden on his family, and nobody can do anything about it. It all clearly takes its tole on them, to the point that Gilbert becomes frustrated enough to slap him.