The usual. Why is Billy the only entity (human or otherwise) who doesn't crap his trousers when Casper shows up? There's a fascinating possible plot here: the weird kid who loves monster movies is rejected by the other tots and constantly told that there's something wrong with him by adults... then a benign spirit shows up, and he's the only one who reacts with, "Cool! A ghost!" That could be a whole series exploring what sort of personality would befriend Casper