We did a lot of "homework" before going to US and IOA. I think it really enriches the experience if you know about the characters, story, etc. Then you can appreciate all the little touches!
So, not only did we watch Harry Potter movies, we watched Jaws and Jurassic park and The Mummy. I found Dudley do Right and Popeye cartoons online. Of course you have to gage which items are appropriate for little kids! But I think know more about the story behind the attraction is very helpful. (BTW, my DD finds HP movie 2 to be the scariest. We like movie Prisoner of Azkaban the best.)