Bobbie Koshay was Judy's double. She is actually seen on film in several key instances: falling into the pigpen, the famous shot opening the farmhouse door into Munchkinland (it's Bobbie, wearing Sepia-tone makeup and dress who opens the door, steps out of the frame, then Judy, in a color dress, walks through the door in the same shot), and Bobbie also is lifted off the ground by the flying monkeys.
Caren Marsh-Doll was Judy's stand-in. She spent many hours under the hot lights while they tested the Technicolor film exposure using a "lily." She also rehearsed all of the dancing and the Munchkinland sequence with the Munchkins for days, plotting it all out with the cameras, before Judy stepped in and filmed it. Caren also spent quite a while testing all of the tornado sequence staging with the giant fans. It was filmed and reviewed before they shot the final footage with Judy, so they could test what worked and what didn't. A gingham dress and ruby slippers were made for Caren as well, since she was used for so much of the testing and lighting. But Caren does not appear in the final film, like Bobbie Koshay.
That's the difference between a stand-in and a double (at least in this case).
"Jaws is the Citizen Kane of movies."
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