I watched the Disney short "Brave Little Tailor" for the first time the other day, and one sequence clearly stood out with complex animation and great appeal. It was the sequence in which Mickey tells the king his tale of bravery. I later discovered it was animated by Frank Thomas, which explains the outstanding quality.
I was inspired to do a classic animation exercise with it. Here's what I did: I quick-sketched about 20 drawings that seemed to be the major key drawing (they are the ones with the circled frame numbers). Then, I animated the scene according to theses keys, without looking at the original scene again. The idea was to compare my results with the original animation, and see what I might have done better. It was a great exercise and I feel I learned a lot from it. The result also shows, I think, how solidly you can define 4 seconds of animation with just 20 very quick sketches.
Note: YouTube is apparently not so good with short videos like this (it won't play it through. To get a better look at it, try the mp4 file here.