A good writer doesn’t just think, and then write down what he thought, as a sort of transcript. A good writer will almost always discover new things in the process of writing. And there is, as far as I know, no substitute for this kind of discovery.
Talking about your ideas with other people is a good way to develop them. But even after doing this, you’ll find you still discover new things when you sit down to write. There is a kind of thinking that can only be done by writing.