A few years ago celebrated fantasy writer Terry Pratchett declared BBC's Doctor Who was ludicrously unrealistic and not real science fiction. Although his Discworld series is set on a flat world sitting atop four elephants balanced on the back of giant tortoise (!) travelling through space it adjusts for a set of rules (see the 'Science of Discworld' series) and observes cause and effect. Similarly, in this Microsoft Research talk, Princeton professor Charles Adler deconstructs what is and is not plausible in the worlds of Harry Potter or Avatar. You can buy the book the talk is based on but this video is free.

http://iti.ms/1qRcIAz