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 effectively highlights Casey's lighthearted nature while also emphasizing her seriousness about the situation, providing insight into her character as someone who enjoys humor but is also capable of being earnest when necessary.