I'm assuming Disney is still using many of the techniques originated by Gary Izzo who first brought this type of interaction to Walt Disney World. (If you are ever *really* interested in this his book while out of print can be found here: http://www.amazon.com/The-Art-Play-I...ords=gary+izzo) (I would kill to see these characters "sheets" as described in the book) (Interestingly enough Gary originates from faire)
These type of interactive characters are built on needs, virtues and foibles.
Often much of the interaction is based on these, does the audience member/guest have something the character needs? It could be the way you stand, something you are wearing...
A interaction could be that you are the "type" that the character and actor interacts with best. I play a poet. In this capacity I am looking for guests to "woo". I've built other "roles" into the character as well, but most often this is my target.
Another trigger is how open is the guest to interaction. The actors pay close attention to the attitudes of their audience. A person refusing to give *any* eye contact may only be given a small test interaction. The character may try harder if the guest is shy and gives quick eye contact.
Hostile interactions are generally kept short, as a fan you *might* want to help by giving the actor an exit if possible :-D