The Disney Characters dress up for Formal and Semi Formal Nights, and meet throughout the evening in the Lobby Atrium and outside the Walt Disney Theater. Check your Personal Navigator onboard for details.
You should certainly dress up for these evenings. Although you won't be told off or thrown out of the restaurant if you don't, you will certainly stand out and maybe get some strange looks! You don't have to go over the top, just black trousers (pants to Americans!), and a shirt for the guys, and a nice dress for the ladies.