The dialogue that would best help develop Casey's character as a lighthearted, joking person 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 showcases her playful nature while also conveying a sense of seriousness about the situation, allowing readers to see both sides of her personality.