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 her seriousness about the situation. It illustrates her personality clearly, allowing readers to understand both her tendency to joke and the genuine concern she feels in this instance.