In James Meek's new novel We Are Now Beginning Our Descent war correspondent Adam Kellas distinguishes between two types of writer: the Bard and the Priest. The one tells stories while the other sells ideas. Each in some way yearns to be like the other.
I've tended to see this slightly differently.
Many of the writers I admire appear to possess two fundamentally different natures, which I shall call their fictional self and their non-fiction self.
For instance, men like Saramago and García Márquez are both Marxists of the rabid persuasion in their non-fictional modes, but men of great compassion and multi-dimensional wisdom once they settle down to pen fictional realities.
I can see this in my own blogging persona. Of course one wants to have lots of slightly beserk nose-ascending opinions to deploy electronically every day, but one also has to feed the perspective engine chugging away in the background, because all good writers know that opinions are just a toe-dip into something deeper.
Yet there are some (good) writers I can think of, such as Richard Dawkins and AA Gill where the non-fictional, idea-selling urge dominates to such an extent that their compassionate novelistic side has atrophied.