In this interview from The Paris Review, he explains the difference:
When you are standing in the pulpit, you must sound as though you know what you’re talking about. When you’re writing, you’re trying to find out something which you don’t know. The whole language of writing for me is finding out what you don’t want to know, what you don’t want to find out. But something forces you to anyway.
It’s all communication, one way or the other.