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 line emphasizes Casey's lighthearted nature while showing that she's serious in this situation, providing depth to her character by highlighting her ability to balance humor with sincerity.