I watched Under the Skin the other week. It was worth a watch, because apart from anything else it is beautifully well-made: it's like the best put-together music video you've ever seen. But it's ultimately unsatisfactory because at a certain stage about two-thirds of the way through it steadily becomes ridiculous. I wouldn't want to reveal spoilers, but people who've seen it will probably know what I mean. It unravels from being an interesting and creepy film about an alien abducting men for some unknown purpose and turns into, "No, I don't buy it" - chiefly because the way the real people in the film behave is totally unbelievable.
An SF story can have weird aliens and incomprehensible technology and things can happen which are literally impossible, but it is unforgivable when the human beings involved don't act like real human beings would when interacting with those aliens/technology/literally impossible things. Doubly so when you consider that the raison d'etre of most SF is to think about what it means to be human by imagining how humans would behave if we postulate aliens or light-speed or time-travel or whatever.
The same is true of fantasy. I can forgive a fantasy book any sin when it comes to letting imagination run wild. But I can't accept it when what the characters do does not ring true given the circumstances. You can come up with any magical spell or bizarre monster or physical impossibility you like, but the reader knows human beings, so woe betide you if you don't get that right.
In fact, I would go as far as to say that most of the SF and fantasy books I've read which I think have failed have been those in which the behaviour of the characters is not credible based on what I know and understand about human beings and the way they act.