Seajam.....you are right, you have to present your actual Disney Visa card. They verify it while you are in line.
We had Goofy & Mickey when we went (and of course, characters can change on a daily basis). It's in a room next to the segway area, and you don't know who is there until you step through the curtain. What is really cool about it is that they take time with whoever is there, and don't rush people through like they do at some other character signings. I know my dd ran up to Mickey, and it took a bit for her to release her bear hug on him. Then they signed the book, and played around her for a bit.