IIRC, they once got some actual SF writers to do a "V" novel, and poor dears, they were frantically jumping through hoops to try to make the scenario make sense. They eventually explained the fact that the lizards could eat Terran meat (along with being reptilian bipeds) with the concept that the lizards weren't actually aliens, they were DINOSAURS, who had left Earth millions of years ago, and who had just now come back.

This is why you don't get SF authors to do TV novelizations: it leads to psychotic breaks.