The very popular characters (the fab 5, princesses, fairies) all have long lines. As nice as it was in the past to have characters wandering around, I do not believe that this would work with the crowds today. As annoying as the lines are, it does prevent your little one from being mowed down by rude, thoughtless adults as they push their little princess in front of her. Now, even shy, well behaved children get to see these characters
While wandering around, I have seen other characters in out of the way places. (Wicked step mom and sisters, good fairies)
You can get a pretty good schedule of these from Kenny the pirate website.