It will be sold out. We went to one that didn't sell out and the lines for the rides were very short. The same is true for the ones that did sell out during our time there. The Candy Lines are long but they move fast. The characters take some time. My advice is to begin in tunetown if characters are your goal and hit that and then the trail to Tomorrowland. Loop back into Fantasyland.
If you're looking for a villains show that isn't that crowded, I can't help you. Every last one of em was crowded on our day. The first parade was absolutely packed. I'm glad we skipped it and went to the second one.
We walked up about 15 minuets before the fireworks and got a prime spot in the hub. You should have no trouble there. We then stayed for the Villains show and watched the second parade from there. It was quite easy.
They'll probably have more rare characters out for Halloween. But I was disappointed in the character mix we got. But all in all it was a great time. Get yourself a costume and dress up. It's a load of fun. I'm sure they'll give out tons of candy on Halloween.