The best piece of dialogue to develop Casey's lighthearted 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 dialogue captures Casey's tendency to joke around while also conveying her sincerity about the situation. It shows that she has a playful side but is also serious when necessary, adding depth to her character.