We walked up 10 minutes before the 5:00 performance last Thursday and got excellent seats but we may have just been very lucky. The line for reserved seating was really lengthy for quite a long while beforehand and the standby line had a lot in it, too...although not nearly as long a line as reserved seating.
We had decided to sit at an outside table at Liberty Inn near the back of the amphitheater to watch but, after all the standby line had been let in, there were many good seats still available. After we saw that, we got up and walked right in, as did many other people. We were pleasantly surprised to be able to do this but the closer it gets to Christmas, I would imagine the chance of this happening would not be great.