Bill touches up his rock because he sees something that needs to be fixed.
A painting has to “make sense”. An obvious example would be a missing shadow that should be at the base of a tree. In this case, it’s such a tiny thing. Most inexperienced artists would leave it out and their painting would be O.K. But Bill sees that it needs to be there! It’s these tiny details that master artists include in their paintings. It’s details like these that make paintings sell!
Always liked his showes. I love to paint now.