In the Star Wars films, David Prowse provided the body for Darth Vader, but he didn't get to give the character his iconic voice.  I believe Prowse's contribution shouldn't be underestimated as Vader's body language is just as important as James Earl Jones' deep, threatening bass.  However, when they were filming Star Wars: A New Hope (and yes, I know "A New Hope" wasn't part of the title at the time), the filmmakers had no idea how they would handle the voice.  Eventually, they landed on Jones, but Prowse was the one who did line readings on set, and it's pretty funny to hear his voice coming from Darth Vader.

This video is a little old, but in case you haven't seen it, it's definitely worth your time.  Hit the jump to check out Prowse providing the voice for Vader.

Via Cool Material.

And as entertaining as I find this video, it doesn't touch this one when it comes to dubbing Darth Vader:

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Image via Lucasfilm