Creator Killer: After previously composing the The Lion King, the disappointing performing of this film botched the collaboration of composer Elton John and lyricist Tim Rice. They have not collaborated on anything since.
Enforced Method Acting: In a good way—Kenneth Branagh and Kevin Kline were actually in the recording booth together (which is rare; actors usually record their lines alone with someone else filling in the other characters) so they could play off each other and banter. It worked beautifully.
Non-Singing Voice: Averted; the main characters have the same voice actors for speaking and singing.
What Could Have Been: If this film had made its money back, we could have a whole franchise of them by now. They deliberately sprinkled the film with sequel hooks, and were already in the early stages of plotting out the sequels. One alternative ending to the film actually had them In Medias Res in their next adventure.
Supposedly, the original script would also have Miguel and Tulio's Ambiguously GayHo Yay played up more with them behaving more blatantly like a gay couple. "Suits" intervention resulted in the birth of Chel's character. What the animators intended is still clearly visible in the actual movie. Many scenes in which the boys act like the couple they were supposed to be were kept and Chel's part has no real influence on the story. In some scenes one can clearly see that she was later added such as when her shading doesn't quite fit.
It also was going to be revealed at the end that the armadillo was an angel/spirit sent to help Miguel and Tulio. Makes sense when you think about it, considering all the crazy stuff that happens with it around, including pretty much flying through the ball hoop.
The movie itself was originally gonna be a Rated R movie.
Chel was going to be a designated sacrifice victim, but the writers didn't want to show the city too soon.