The dialogue that would best help develop Casey's character as lighthearted and humorous, while still conveying the seriousness of the situation, 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 balances her playful nature with the gravity of the situation, emphasizing that even though she enjoys joking, this is a legitimate concern for her.