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 providing a serious moment, showing that she is earnest despite her usual tendency to joke around. It allows readers to see her personality and the contrast in how she normally behaves versus the seriousness of the situation.