The piece of dialogue that would best help develop the character of Casey as a lighthearted character who likes to joke 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 captures Casey's playful nature while also highlighting the seriousness of the situation, showing a balance in her character. It indicates that while she typically enjoys humor, she is sincere about this concern, illustrating her character more vividly for the readers.