The piece of dialogue that would best help develop Casey's character as a lighthearted, joking individual 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 captures Casey's tendency to joke while also conveying the seriousness of the current situation, highlighting her frustration at having to assert that she's not joking this time. It shows her personality well, balancing her lighthearted nature with the urgency of the moment.