The piece of dialogue that would best help develop the character of Casey 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 dialogue effectively shows Casey's lighthearted nature with her admission about joking around, while also indicating that she is serious in this instance. It balances her personality and the need to communicate a genuine concern, allowing readers to understand her character better.