Engender
Another subject in writing that can be as imagination destroying as the technique of characters describing themselves through a mirror is the idea that you can't write outside your gender. This would also extend to ethnicity, class and species - if Orwell can put himself in the head of talking animals, why should it be impossible for a woman to do so with a man? Or vice versa? I see warnings against this every so often. There was a little bit of this when 'The Help' came out (the book), and some reviewers expressed concern with the author writing in the voice and from the perspective of a black woman. A failure to place yourself as a writer in the mind of someone utterly apart from you in background or biology is a failure of imagination.