The piece of dialogue that would best help develop Casey's character as a lighthearted individual who enjoys joking around 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 emphasizes Casey's playful nature while also showing her seriousness about the situation, which aligns with her character description.