If you know much about Milt then you know he thumbnailed his scenes extensively. Supposedly he would try every way he could think of to do the scene; experimenting with different poses and expressions and attitudes. In one of the audio clips of him floating around on the web (I got it either from Jim Hull's site or The Animation Podcast) I think he talked about how he tried every variation of pose and attitude in the thumbnail stage. Then he would know exactly what worked and what didn't. In the course of his lecture he implied that he didn't have much use for comments from others about how to improve his scenes. After all, he had already tried everything in the thumbnail stage and figured out which idea was best. Anyone criticizing his work off the top of their head hadn't put as much thought into it as he had!
I thumbnail every sequence I storyboard for the same reason. Thumbnailing is a fast and easy way to try out different acting poses, layouts, compositions...everything that makes for a good sequence. If you don't thumbnail then you are just settling for the first idea out of your head. This works occasionally, but most of the time you want to dig deeper than your first idea. Just like Milt, you should strive for the best way to do it. Unlike Milt you should always be open to criticism about how your work could be improved (unless you happen to be a genius). If you don't thumbnail and you just jump in and start working you will undoubtably run into problems in the middle of your scene or sequence because you encounter a problem you didn't anticipate. Then you won't know how to get out of it!
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