The piece of dialogue that would best help develop Casey's character as lighthearted and playful 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 captures her humorous nature but also highlights that she is serious about the mouse issue, making it a well-rounded representation of her character.