As the most creative, literally, of all genres, science fiction places a tremendous burden on the writer. You have to create the world, in detail. Children of Men gets some of this right. But there's a lot missing, too.
The first rule of science fiction is to remember that you're writing about the present world, not the future. Indeed, the biggest mistake science fiction writers make is they place the story in such a bizarre and unrecognizable world that the audience can't identify with it. The viewer takes a clinical attitude toward the story, and all emotion is gone.
The world of Children of Men is just weird enough to feel like the future, but it is also horrifyingly present tense. Immigrants are outcasts, torture is justified, bombs explode anywhere for no apparent reason.
It is in the details, however, that Children of Men runs into problems. The most important detail of any science fiction world is the basic rules by which the society works, and that is not clear here. Why all the deportations? Why the war with the immigrants? Why is a baby born to this woman and no others? I can guess at these things. But the writer doesn't want the audience to guess, because that means they are thinking about the construction of the story and not the story itself.
Children of Men also fails to explain or justify the desire line. This is the motive for the quest. Everything else relies on it. So it must be very strong and completely believable. Why is this guy facing almost certain death to escort a woman he doesn't know? Telling us he needs money and is a former activist doesn't cut it. Why does the woman have to go on this trek at all? Saying the British government won't recognize a baby born to an immigrant woman is absurd.
And that leads to the biggest problem with the script, one that is very common to science fiction stories. Science fiction often borrows the journey technique from the myth genre in order to structure the story. The problem with the journey is that it can easily become episodic. The hero encounters a series of opponents on the road but each attack is essentially the same beat. Which means that this kind of science fiction story has no plot. Sure enough the plot in Children of Men quickly becomes tiresome. The two leads drive for a while and then get attacked. Then they drive some more and get attacked again.
This film has some amazing cinematography that goes a long way toward making this world seem intensely real. There is a single take during the battle scene - about ten minutes long - that will take your breath away. But ultimately any film, including science fiction, comes down to story. If you don't set up the story world and the plot properly, no amount of camera work will cover the holes.
If you would like to learn all the techniques for setting up a detailed world and a plot that builds steadily to the end, take a look at the Horror, Fantasy, and Science Fiction Class or the Science Fiction add-on to Blockbuster.
