Well, my DD is a 'face' character and I know they love to have special questions asked of them.
Ask Snow White:
--about the Dwarfs...
--what kind of pie she bakes for them,
--what the Dwarfs are doing in the mine etc;
Ask Ariel:
--about Sebastian, Flounder and her Daddy, King Trition.
-- how she gets her 'human legs' for the afternoon parade.
As for what NOT to say or do...
Please don't complain to the characters about your long 'wait time'--it isn't their fault; they try to make each child feel 'special' and sometimes that takes a while.
Please don't yell, scream and curse at them if they get pulled 'off stage' just as it's your turn to talk to them. I know that's incredibly frustrating and disappointing but again, this is beyond the characters control. They have to follow the rules--if they don't they can get in BIG trouble.
If you're a man, please don't talk to the characters about their physical attributes!!!!!!!!!! Don't mention how 'well blessed' they are or discuss bra size!!!!!!!!!This is a BIG problem for both Ariel and Jasmine. They don't find it amusing but they are not allowed to be rude to a guest .

Also, keep your hands to yourself!!!!!!!!!!!!!!!!
Don't purposely insult the character or her/his appearance. Don't say she/he is UGLY or FAT!! Yes, it happens!!!!!!! Try to remember the people inside those costumes may portray cartoon characters but they are human beings and they have feelings too!
Don't go near the characters when you're smoking. (Yes, I know smokers should be in special smoking areas but many times people break the rules). Some of the characters have asthma or other pulmonary problems and having smoke blown in their face can be a major problem for them.
The characters --both face and fur--love their jobs and they work very hard . Say THANK YOU!! It 'makes their day' to know they've brought some 'magic' into your life.