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 highlights Casey's lighthearted nature while also showing that she is serious about the situation, which adds depth to her character. It reinforces her tendency to joke around but clearly illustrates that she is not taking the mouse sighting lightly, providing a balance between her playful demeanor and the gravity of the issue at hand.