For this type of party, you can really go all-out in both costume and food. Make sure each player has a brief description of their character they will be playing and have them arrive in full persona mode both in costume and attitude. I can go into detail about the type of food you can serve but, to be honest, the Night of Mystery really does a fantastic job on telling you how to set the stage with backdrops and foods that fit your theme.
http://www.nightofmystery.com/
http://www.mymysteryparty.com/