Jamaal Bradley posted a couple of pages of his thinking and planning for a great shot he did in Tangled. I usually thumbnail before I do anything else. Video reference is helpful, but thumbnailing always comes first. Part of my workflow used to include an animated thumbnail pass before doing the animation at full size. It's all about thinking through your ideas and working towards a visualization of the scene before you commit to going in and doing the actual motion that the audience ends up seeing. These pages certainly fit the bill. They show thought and exploration. It's great to compare his initial thoughts with the final product (which you can see here). A lot can change from a first thumbnail pass to a final animation, but in my experience and as seen in Jamaal's sketches, there is always a strong connection between my final animation and the first burst of ideas and problem solving in a small series of doodles.
Here is a small sheet of thumbnails that I did. I never animated it, but I still see all the ideas I wanted to hit in the animation. Usually when I get into the weeds of planning all the movement, my thumbnails get a lot more scribbly and off model. But as long as it shows the ups and downs, and the rhythm of the motion, it provides a perfect road map for diving into the animation.
And here is a random sketch I did. Don't ask me what it has to do with this post.