We're actively encouraging people to dress up...it's kind of like a big fancy dress party with paintball guns
Unless you're an E-jock in which case you probably don't actually need a costume...well it IS cold in that part of the world...
As for specific characters they'll be identified individually. Of course if you look just like "THE" Han Solo you may find yourself getting harrassed by bounty hunters all day...who lets face it are not too bright and will probably shoot first and see if they can think of any questions to ask later...