It's simple, sentences or something they would never say.
Something that would destroy the idea of their character, if they said that is would change who they are as a character.
(Not exactly what they leave unsaid, just stuff they wouldn't say in general. Though it could be that too.)
Chester:
- "Can I kiss you?"
- "My life is worth something."
- "I love myself."
Harli:
- anything at all
Oscar:
- "Dad, can I go outside?"
- "I'm not okay.."
- "I wanna die."
Landers:
- "It is against my protocol."
- "I'm gonna kill you."
- "I hate hamburgers."
Lincoln:
- "I was exposed to bad chemicals as a child, they made my brain all funny."
- "I don't need a goal."
- "Yo, dude, bro, hey, man."
The Prince:
- "Father was wrong."
- "Children make me happy."
- "What a world to live in, it's so beautiful."