You can walk up at show time if you like. Last October we were rushing from somewhere to catch the show, and were able to get a seat 5 min. after the show started. They took us in a different entrance to fill in one section of the theater. Easter week we lined up about 10 min. before the show, and had no trouble getting a seat. I don't recall that the theater was even filled...close, but I think they took some late comers.
I'd like to say that the earlier you get there the better choice of seats you have, but I'm not sure that is 100% true (it was the first time we saw it. Lined up about 25 min. before the show). They seem to try and distribute the crowd among the four sections, rather than guests just picking a spot. Since we pretty much walk up a few minutes before show time and simply go where they tell us to. Over Memorial Day weekend we were instructed to sit right up front to fill in a section.
The show lasts about 25 min. and well worth every second you'll spend there.