This video is from the National Gallery via YouTube posted by Jeffrey Collins. (It says footage of Guston from Michael Blackwood.) I took it off Facebook courtesy of Tracey Physioc Brackett.
It's fascinating to see him working so loosely. His paint handling is just fabulous. But even more important is his struggle to be true to himself and his ideas.
If you are interested in Guston, you might want to read my earlier posts about his life and work: