I went to a Picasso exhibition in Madrid once where they had an entire, huge, long room filled with every sketch and painting he'd done in preparation for Guernica, plus Guernica itself of course. It was eye-opening to say the least, especially as so many of the prep-work pieces were not in his typical style. Some were in an absolutely beautifully detailed realist style, and at that point I'd never seen a Picasso that wasn't cubist so it really stuck with me.