There tends to be an assumption among both filmgoers and filmmakers that different genres of film are held to different standards of believability. If we were to put them in descending order, from most realistic to least, I imagine the results would look roughly like this:
2. Romantic comedy
Sound about right? We're demanding a lot more from Ordinary People or Slumdog Millionaire in terms of believability than we are from Lord of the Rings or Star Wars, whereas Saw or Ocean's Eleven can get away with a decent amount but not too much. We can quibble about the specific order of the list -- maybe horror films have gotten more realistic over the years, while rom-coms have gotten more fantastic -- but that's really not my point here.
My point is that the list is irrelevant -- and internalizing this sliding-scale system of realism can only hurt us as screenwriters, especially when we choose to write scripts that fall toward the bottom of the scale. We can trick ourselves into thinking that we can get away with all sorts of things just because we're writing in a "less believable" genre -- but that's not the case.
Truth applies no matter what, and it must be sacred regardless of genre.
OK, sorry -- that was a little Robert McKee of me, but I promise it was for a good reason. Because this concept isn't just something to worry about; it can also be very helpful. It's a powerful beacon that can guide us through the writing of some really out-there material -- enabling us to take the most far-fetched story imaginable and make it universally appealing and meaningful. It's what makes District 9, a shockingly violent and gory film about repulsive aliens, one of the most heartrending movies of the year (not to mention financially successful and very well-reviewed): every single moment of it rings true.
It's what accounts for the difference between a great superhero movie like Spiderman 2 and a terrible one like Batman & Robin. The former imagines what it would be really like to be a superhero -- focusing on all the worst parts of it -- while the latter uses the superhero/supervillain backdrop as an excuse to dump a bunch of visual nonsense on the audience, never showing us a single character with any recognizable human qualities. B&R progresses arbitrarily from ridiculous set piece to set piece; S2 spins a hugely entertaining story (with no shortage of amazing set pieces) that evolves organically out of decisions made by actual people, all of whom we sympathize with or at least understand.
The screenwriters who worked on Spiderman 2 had the unenviable task of taking a fairly absurd premise -- Web-slinging teenager takes on eight-limbed mad scientist -- and asking themselves, "What would this really be like? How would people really feel in that situation? What would they really do in response?" There's no trick to solving those problems. They're like multiple-choice SAT questions -- there's no right answer, but there's a best answer... and a bunch of wrong ones. Our goal as writers is to weed out the bad choices and zero in on the one that makes the most sense.
It's not easy.
It requires that we reject all the easy, facile solutions that pop up in our brains ("What if the hero just kind of stumbles into the control room where he can defuse the bomb?"). It demands that we crawl inside the minds of our heroes, villains, supporting cast -- even the ice cream cone guy who just has that one line -- and come out with truth. We may not know how to get there, but we always know when we've found it. So will our readers and audiences.