The piece of dialogue that would best help develop Casey's character as a lighthearted person 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 captures Casey's lighthearted nature while also highlighting the seriousness of the situation, showing that even when she's being playful, she has moments where she can be earnest, making her character more relatable and multidimensional.