I think we actually answered this person's question, but since I know that others might be confused, I'm going to try to answer it more completely.
There are 3 main stage shows on the Magic and Wonder. 1) Hercules, 2) Disney Dreams, and 3) Golden Mickeys. Depending on the length of your cruise, you may also see one or two variety shows, Who Wants to be a mouseketeer? or a "PreEar at Sea" in the Walt Disney theater. Each show is shown one night only. There are two or three seatings (depending on whether they have a matinee). These seatings are based on your dining seating. If you have late seating for dinner, you go to the 6:30 show. If you have early seating for dinner, you go to the 8:30 show. As the previous poster said, they don't check to be sure you're not "crashing" the other show, it's just that if you're at dinner, you can't possibly go to the other show. You would only go to the other seating's show if you ate at Topsiders, Palo, room service, or didn't eat at all that night. Your navigator will tell you what the show is for the night, and if they have a matinee.
So if you miss Hercules on the night it's shown, you wouldn't be able to go a different night to see it.
The movies (with the exception of the PreEar at sea) are held in the Buena Vista theater. Show times are listed in the navigator, and I think there's also a listing of the day's movies on one of the TV channels.
All shows are open seating, regardless of the theater. If you want a good seat, come a bit early.