The dialogue that would best 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 piece of dialogue highlights Casey's lighthearted nature by acknowledging her tendency to joke, while also emphasizing the seriousness of the situation, providing a contrast that deepens her character.