Kenny the Pirate has a character locator you can subscribe to for a small fee, with very good information on where and when characters are appearing each day. As mentioned above nothing is guaranteed and can change day to day, but you can get a good idea of who may be appearing for your plans, and can check the night before/day of for any changes. It also includes wait times, crowd calendars, etc. Steve Soares' site (wdwent.com) also posts weekly schedules for each park with showtimes and characters - again can use ahead for general planning, and check back week of - not as detailed for characters, but available for free.