The 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 response captures her lighthearted nature while also revealing a moment of seriousness. It shows the contrast between her usual joking demeanor and the genuine concern she has in this situation, allowing readers to see both sides of her character.