The 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 line effectively establishes that Casey has a playful personality while also showing a moment of seriousness, indicating the character's ability to switch between humor and sincerity. It reveals that Casey is not someone who always takes things at face value, which could add depth to their character.