When getting into character, Leonardo DiCaprio remarked of it, "I had to really research to get into the mind of somebody like that. I spent a few days at a home for mentally retarded teens. We just talked and I watched their mannerisms. People have these expectations that mentally retarded children are really crazy, but that isn't so. It's refreshing to see them because everything's so new to them."
Darlene Cates had not left her house in five years at the time of the film's release. Some of Bonnie's problems mirrored those in Cates' life, e.g., people staring at her, teasing her, and retreating from wide society.
Johnny Depp felt bad about having to ridicule Darlene Cates' character, "Momma," and would often apologize to her after shooting. Depp once said to Cates, "Man, I want you to know how much I hated having to say those things about you, or about your character."
Despite the fact that animal protection agents were on set to monitor the well-being of the grasshopper Arnie decapitates in the letterbox, Leonardo DiCaprio actually ended up killing it, accidentally.
During the scene where Darlene Cates (Momma) climbed the staircase, she climbed only the first two or three steps. This footage was reused to show her climbing the entire staircase. The room that served as her upstairs bedroom was actually a downstairs room.
Johnny Depp wanted to play Gilbert Grape as someone who seems constantly half asleep/half awake--someone numb to the peculiarities in his life. Juliette Lewis completely agreed with his interpretation of the character. Even still, Depp worried he was boring by playing Gilbert so mildly. Lasse Hallström, however, agreed with Lewis, and thought that Depp nailed the part.
Arnie Grape originally was not supposed to be such a big character in the movie as he is, Leonardo DiCaprio turned Arnie into this really happy, excited character and really made Arnie stand out and stole the show.
Leonardo DiCaprio played the part of Arnie Grape so well that when he showed up for the film's premiere, many people were shocked and astonished to learn that the young actor was not actually mentally disabled.
Johnny Depp admitted that he got easily irritated by co-star Leonardo DiCaprio during the filming of the movie. In an interview, Depp said, "It was a hard time for me, that film, for some reason. I don't know why. I tortured him (Leonardo DiCaprio). I really did. He was always talking about these video games, you know? I told you it was kind of a dark period... 'No, I will not give you a drag of my cigarette while you hide from your mother again, Leo.'" Depp concluded, "I'd say the absolute truth is that I respect Leo a lot. He worked really hard on that film and spent a lot of time researching. He came to set and he was ready to work hard and all his ducks were in a row."
John C. Reilly and Crispin Glover both appeared together in 9 (2009), where Reilly played 5 and Glover played 6. They also appeared together in an episode of Drunk History (2007), where Reilly played Nikola Tesla and Glover played Thomas Edison.