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Anakin was originally supposed to just watch the entire fight between Palpatine and the other Jedi Masters, with Palpatine even having stolen Anakin's lightsaber to do so. The entire fight would have had Anakin debating on which side he was going to choose. They even filmed it, but they figured that Anakin simply watching the fight meant that he had already made his choice, so it was re-shot. Further, the final fight between Windu and Palpatine was supposed to be an all-over-the-place masterpiece, but due to George Lucas wanting Ian McDiarmid to do as many of his own stunts as possible, it was reduced to, largely, Windu forcing Palpatine down the hallway and then a bit of a scrap in the office before Anakin showed up and both started talking to him.
George Lucas deliberately made the Darth Vader suit top-heavy (for instance adding weight on the helmet) to make Hayden Christensen not appear "too accustomed" to it in the movie.
The original cut of this movie ran nearly four hours. The opening battle/Palpatine rescue alone ran over an hour. The extra footage of the Palpatine rescue scene is shown in the video game for this movie, however.
Ewan McGregor and Hayden Christensen trained for two months in fencing and fitness in preparation for their epic battle. As a result of their practice, the speed at which Kenobi and Vader engage the duel (in the completed movie) is the speed in which it was filmed, and was not digitally accelerated.
The images of the volcanic eruption on Mustafar is real footage of Mt. Etna in Italy, which was erupting at the time of production.
Apart from providing the voice of R2-D2, and the heavy breathing of Darth Vader, which he has done since Star Wars: Episode IV - A New Hope (1977), Ben Burtt provided the voices for every Battle Droid, Super Battle Droid, and Buzz Droid.
Jett Lucas: The young Jedi who rushes from the Temple towards Bail Organa's speeder during the Jedi Purge is played by George Lucas' son.
Nick Gillard: Stunt coordinator as Cin Drallig, a Jedi Anakin kills in the video recording Obi-Wan watches.
George Lucas: The sound of General Grievous' coughing is George Lucas' own coughing. After developing a bad cough during production, Lucas had it recorded and used as Grievous' own cough.
George Lucas: The blue skinned Baron Papanoida who appears just outside the entrance to Palpatine's private box at the Galaxies Opera House.