The piece of dialogue that would best help develop Casey's character for readers 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 dialogue effectively highlights Casey's lighthearted nature while also conveying the seriousness of the situation. It shows her usual playful demeanor but contrasts it with a moment of sincerity, which gives depth to her character.