The 1950 Disney film Cinderella was produced at a time when Disney was struggling financially after World War II, so they cut corners every way they could. They had used reference films for earlier movies, but every scene in Cinderella was filmed with actors in costume for reference, which the animators copied and were told not to stray from. You can see some footage in this video, starting at 1:45. Still images from the original production are more numerous.
Learn some of the behind-the-scenes trivia about the production of Cinderella in a pictofacts list at Cracked.