The best piece of dialogue that would help develop Casey's character for readers is:
“I know I like to joke around a lot, but I’m not joking this time,” Casey said. “I saw the mouse myself, and it’s been eating things and running around in the walls.”
This line highlights Casey's lighthearted nature while contrasting it with her serious claim about the mouse, showcasing her character's humor and earnestness in a situation that might otherwise seem trivial.