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The concept is that the mechanical helmet processes his air and the "breathing" you hear is the release or admission of air.
No, he only dubbed the voice. The person you see in the film is actor David Prowse. David Prowse said in an interview that he was shocked to go see the film and not hear his own voice. He had not...
Helmet mask cape light saber
According to Wookieepedia, Darth Vader's armor is 2.02 meters tall (6 feet 7 inches)