For those of you who are hoping to walk-up, you have a good chance of getting in. Stayed at Riviera last month.
Dinner - Changed my mind for arrival day and stalked the Reservations site. Easily found a dinner reservation the day before arrival day (so don't lose hope). I keep typing this, but it's true. People are holding multiple reservations per meal period and canceling right before the 24 hr mark. As such, you have a good chance of getting even hard-to-get reservations within 24 hrs. Noticed that there was a sign at the host stand that said "accepting walk-ups" when we arrived to dine. I believe the bar / lounge section is first-come, first-serve. There was space in the restaurant throughout the night - meaning empty tables both in the dining room & at the bar. Not sure if they hold a certain # for walk-ups.
Breakfast - Kept my reservation that I booked 60 days out. Again, sign was up stating "accepting walk-ups" when we arrived at the host stand. We overheard the host mentioning to people that they were welcome to sit at the bar / in the lounge. The prix fixe price and menus were the SAME in the bar / lounge as they were in the dining room. However, the characters do NOT go to this section to say "hi" from a distance. You can still see the whole song and dance, but it's at a distance and the characters will likely not face you in the bar / lounge. They will also not pose for photographs (at a distance) for those in the bar / lounge. The hostesses were very good about explaining this, as they should be because you're paying for the full experience but not receiving the character aspect of it. Many adults were fine with this and there were a decent number of adults (couples) sitting at the bar and high-tops in the bar / lounge area. I do not know if they allow children to sit in the bar / lounge area for breakfast. My guess is yes because I saw kids in that area for dinner. However, it likely is such a tease that they're not closer / allowed to participate in the whole show.
Good luck whatever you decide to do. The restaurant is great and it was a nice "new" experience for us. We also LOVE the resort.