I don’t mean to belabour the point, but just in case, have you done Fantasmic before in the past? If you have, skip the rest of this post and go about your day, but if you haven’t, the theatre is divided up into three sections - Fastpass holders on the left, dinner packages etc. in the middle, and stand by on the right. When we went we arrived 30 minutes before showtime and both the fastpass and standby sections were approximately the same amount of full - that is to say that had we not had a fastpass, we would have gotten basically the same seats, only on the right side of the amphitheatre. The real difference was at the end when 7000 people all tried to leave the place at the same time by way of a single exit on the far right of the amphitheatre - we were packed shoulder to shoulder so tightly that you could barely move for at least 20 minutes. So having a fastpass a) doesn’t guarantee you a seat, b) doesn’t allow you to arrive later than you would have to if you didn’t have a fastpass, and c) slows you down considerably when you’re trying to leave - I actually believe that a fastpass for Fantasmic is worse than not having one.
Sorry, I’m a bit fanatical about this because I’m not a huge fan of crowds and trying to get out of Fantasmic was a completely miserable experience.