They have 3 nights where characters are there, not sure of the nights but it was Mon was S&S Wed was The Simpsons and Sat was Shrek and Fiona. Now they apparently had changed the characters around after the summer season, so while the nights where the same ,the characters could be different.
If you Phone RPR directly ,they will tell you who is on and what nights they are there. We always stay at RPR and always have had dinner here the last 5years, usually a few times during our stay when S&S are there, as they are DW & DD fav.
Its just a normal dinner, and the characters are there, so does not cost extra, if you went a night with no characters it would be the same price. Its a lovely restaurant with excellent food and service, have never had to make a reservation, as it is always quite empty, do not know why, and this is July when we are there. Get plenty of time with the characters, when we where there last July, there was only 3 tables with people at them, and S&S spent a good 20 to 30 mins talking to us [well Shaggy did as we know Scobby cannot speak] they also have an area with a kids buffett which is excellent.