Here's a clip of what Barnes has to say, if you want more you'll have to head over to his blog:
do you really believe what you just wrote? Would a real human being, in this situation, really do or say what you just wrote? Even if it’s a fantasy story, would a real being, evolved within a universe of fantastic potential, behave in this fashion?
The post is worth reading for any of you potential authors/screenwriters out there. Let me add that one of the writers recently added to my "it is imperative that I read every book by this author list", John Crowley, has said "I think that if a novel has no whiff at all of the impossible, the fabulous, the inexplicable, the metaphysical as the Romantics meant the word, then it isn't very realistic, because the real (this, our shared physical and biological) world does have those intimations in it. (When the intimations become certainties, you have fantasy.)"