The Japanese version's Mewtwo is shown in a more favorable light, feeling that it has no place in the world and so wants to "earn" its place. The English dub opts to make Mewtwo an emotionless tyrant who wants to destroy the world, a decision that both fans and critics criticize. In hindsight, the producers felt they made the wrong decision.
The Japanese version has an entirely different story.
Producers and director Michael Haigney had trouble matching animated lip movements. The script was written twice to ensure all dialog matched up. They said they were very happy Mewtwo was a psychic Pokémon because his mouth rarely moved, which saved them work for making dialog match his mouth movements.
A narrator and Mewtwo's voice were added in the American version's first segment to help better explain the movie to people who may not have had much knowledge of Pokémon (for example parents who took their children to see the movie).
It held the record for the highest-grossing opening for an animated film that debuted in November outside of the Thanksgiving holiday weekend.