I'd like to know, because it's a phrase that means many things to different people. For example:
- Writers need to maintain an emotional distance from their characters and the situations they're in, so as tell the story that needs to be told from an artistic point of view, even if that means bad things happening to good fictional people.
- Writers are in some respects maimed individuals who leverage their character flaws to create art. The chip is to artists what grit is to oysters.
- Writers cannot be expected to behave decently to those around them because writing is a high and noble calling, etc.
Not that these beliefs need be mutually exclusive. I suspect that Greene had the first in mind, and maybe a bit of the second, and I hope the third not at all, but I've heard number three trotted out too. If, as I suppose, Greene was alluding to "The Snow Queen" he presumably didn't mean it as a compliment to his profession, or not entirely, but self-praise is often wrapped in the loose mantle of self-criticism, as I well know (speaking as someone who's really too self-aware for her own good), and it would be useful to see context of Greene's remark.