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 line demonstrates Casey's lighthearted nature while also highlighting a moment of seriousness, showing that she is genuinely concerned this time despite her usual tendency to joke. This contrast adds depth to her character, making her relatable and believable.