They used to hand out fastpasses in front of the theater for the different show times, But they no longer do this. You just have to line up and wait.
However, if you go during a slow season, you can get a seat 5-10 minutes before showtime. It's a huge theater. But if you'd like good seats, I'd show up alot earlier.
Also, the last show of the day always seems to be the most crowded.