Jim Henson discusses the technique and philosophy behind building and performing Kermit the Frog in a 1990 interview.
YouTube has a segment from a 1990 video of Jim Henson, Frank Oz and Michael Frith discussing the philosophy behind the Muppets. The interview doesn't contain any completely new information, but it's still really interesting and informative to watch.
Thanks to everybody who sent in a link to this!
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30/4/07 Update: This video is from 1990, not 1980 as I originally indicated. Also, it was apparently created for the original Henson/Disney deal which fell through after Jim Henson's death to show Disney merchandisers how to properly present the characters for merchandising purposes.
