When I draw out my thumb nails, I'm trying to think of the action and acting. Not so much about drawing.
I find that if you draw small, it's easier to not get caught up in the drawing for the sake of drawing. I'll fill a page or two with these type of doodles then cut and paste them in sequence onto a final page. This is were you want to let your imagination go wild. Sometimes the poses just come to you and other times you go through many sheets of paper and frustration to find the right poses. Don't know why, just seems to be that way for me. Thumb nails can be looked at as shorthand notes for the writer. I recommend that what ever your small thumb nails look like and if you understand them, use them. Don't try to make pretty drawings for other people to ooooh and aaaah at. These sketches are so YOU can get a better understanding of your scene.
KEYS AND BREAKDOWNS
After you get a clear understanding of your scene from your thumb nails you can start to rough in your scene with KEY drawings and BREAKDOWN drawings. The keys are drawings that tell the essential story of your scene.The breakdown drawings are the ones that tell the rest of the story.
Breakdowns are NOT inbetweens. Breakdowns are drawings that describe the arcs, flow,overshoots,overlapping, follow through and such. INBETWEENS are as they sound. They go inbetween the extreme drawings (keys and breakdowns). That doesn't mean that the inbetweens don't have arcs and the principles of animation, they do. A bad inbetween can ruin a scene. I highly recommend THE ANIMATOR'S SURVIVAL KIT by RICHARD WILLIAMS. If you want to understand animation, this is the book!
The drawings below are the keys and breakdowns from the thumb nails above.
This was not the first pass of the scene. I truly wish I was one of those who could draw a simple under drawing and throw some graphite on top, but I'm not.
Below is the scene with just the Key and Breakdowns
Below is the same scene with inbetweens