The piece of dialogue that would best help develop the character of Casey 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 dialogue effectively balances Casey's lighthearted nature with the seriousness of the situation. By acknowledging her tendency to joke while insisting that she's being sincere this time, it showcases her playful character while also demonstrating her concern about the mouse. This contrast effectively deepens readers' understanding of her personality.