The WDW website says that seating begins 90 minutes prior to Fantasmic but the Fast Pass window is 40 to 25 before the show. How is this reconciled?
There is a separate seating area for FPs, which will be opened up to everyone about 15 minutes before the start. Same for dining packages. So it means you can arrive a bit later. I wouldn't leave it much beyond 30 minutes though, regardless.
I was amazed by how packed it was when I arrived. I had tickets from the dinner deal and arrived about half an hour early. I got a great seat, but then I’m not sure any of the seats are bad, to be honest.
(The blue dot is my seat.)