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 also showing her seriousness about the situation. It captures both her personality and the urgency of the issue at hand, illustrating that she can be playful while still recognizing when something is important.