"Perhaps you do, but-"
A gunshot echoed through the room, and the top of Bond's head evaporated in a spurt of gore. Blood ran from a hole between his eyes, and his dying body fouled his tuxedo pants. The henchman dropped the twitching meat that had formerly been one of MI-6's finest agents, and their leader holstered his compact nine millimeter. They walked away as if nothing had happened, the agent's death already less than a memory.
Said no book ever.
Despite the fact that no book will ever have a scene like this (unless George R. R. Martin starts writing spy thrillers), we've all wondered why this scenario has never happened. The hero is placed in a weak position before a ruthless villain or an unstoppable situation, and instead of letting the story follow the logical flow, the author chose to use a hackneyed trope, a bizarre coincidence, or what can only be described as meta knowledge (something the author or the reader knows, but which the character cannot possibly know) to make the story go a different way.
This is referred to as the "Why Didn't They Just" Clause, because when you finish reading the scene the first thing you're going to ask is, "Why didn't they just," followed by the obvious course of action.
Why Does This Happen?
More often than not the "Why Didn't They Just" Clause crops up when an author wants to achieve tension, but does so in a way that introduces obvious logical problems. For example, the reason none of James Bond's villains put a gun to his head and blow his brains out is because, if that happened, there would be no more story to tell. Your lead is dead, the bad guys won, finito. So to ratchet up the tension villains put James in supposedly inescapable death traps, and then walk away with the understanding that the job is as good as done.
|And now they can steal his sweet, sweet car.|
There's cognitive dissonance there, though. On the one hand the author has told us this villain is ruthless, intelligent, and efficient in order to give our hero something dangerous to oppose. That's why the inescapable death trap trope makes no sense; a canny, successful villain wouldn't believe James was dead until he saw the body. So, to avoid the logic problem you simply need to create another, easier-to-swallow scenario where James can't just be executed. For example, the villain might need to know certain pieces of information that only the secret agent can provide (as we see in Daniel Craig's Casino Royale). Bond may be a valuable bargaining chip, which could be used to procure freedom for other enemy agents. There are all sorts of ways to keep the lead alive, and give him a chance to escape, without violating the "Why Didn't They Just" Clause.
I'll give you another example; one from a book series that is less trope-prone than 007's world.
|It is still British, though.|
The Harry Potter series is about a secret world of magic existing in the modern day. Harry goes to school, and gets embroiled in adventures which culminate in him fighting Grand Dragon Hitler the undying lich-lord of evil. We are repeatedly shown in the later books that the forces of evil and tyranny are smashing the agents of justice, and soon it will come down to a single fight, mano a mano, between Harry and Lord Voldemort.
There's a big hitch, though, and it's one that we're made deliberately aware of in prologue. In a passage which has nothing to do with Harry, and which he can't possibly know about, we see the Minister of Magic speaking with England's Prime Minister. The scene is essentially a summation of how an army of giants, werewolves, witches, and dark wizards are coming out of hiding in the British Isles, and they're fighting a war of conquest that's breaching the traditionally held secret boundary. They're winning, and he makes no bones about how many good guys are dead or dying. The M.o.M. then turns around and buggers off.
Now, the point of this scene is to increase tension, and to show the reader what's really at stake. The non-magical characters, who are arguably far more numerous than the magical ones, are left in the dark, sitting around and waiting for their fates. The first words that went through my head after hearing that was, why didn't the prime minister activate the S.A.S., put MI-6 on the job, and go to war to defend his country?
|I see a wand. Permission to engage, sir?|
The reason that didn't happen is that this book series is not about a bunch of hard cases in black kit going toe-to-toe with giants, werewolves, and a magical hate group for the fate of their nation (more's the pity). The books are about Harry, and he has to be the hero for the book's formula to work. The problem is that by using the tool of "inform the mortal leader how bad things are getting," Rowling left a great, big logic gap in the center of her story. If you cut that prologue out it doesn't change the book in any significant way, and it eliminates the cognitive dissonance that will drag across the reader's mind like nails on a chalkboard.
Common Clause Violations
It's easy to lose your head when your story starts to run away with you. With that said, there are some scenarios that can snap suspension of disbelief if you aren't careful.
1. Calling The Cops: When something bad happens to you, you call the cops and report it. Your wife was murdered? Your house burned down? Someone made a threat against you? The boys in blue are your first port of call. Unless your lead is someone who has compelling reasons not to go to the police (they didn't catch the killer before, lead is a criminal and has to handle it himself, situation came with a "don't call the cops" sticky note that's being followed), that's the logical first step.
2. Being Arrested: The other end of the spectrum is characters whose epilogue should include communal showers and pumping iron in an orange jumpsuit. A one-man vengeance-fueled crusade is pulpy fun at its best, but if you don't explain how your lead got away at the end then there will be a lot of readers scratching their heads and Googling just how many laws he broke by the end of the book. This is doubly important for characters who aren't secret agents or career criminals who have no experience evading the law or covering up crimes.
3. Relationships: No one is good at relationships, contrary to what the gurus might tell you. However, how many times have you watched a series of truly improbable coincidences unfold and just wondered why he didn't call her up and ask for an explanation, or why she didn't come over to his apartment, sit him down, and tell him what was happening and how she feels? If readers start wondering that, the next thing they'll wonder is why they're reading this book.
4. Deus ex Machina: If you've established that a particular plot device (or just a regular old device) exists, and that it can solve certain problems, there is no reason for those problems to go unsolved as long as the device works. For example, if you're being stalked by a crazed killer, and you have a cell phone, 911 is your best friend (see #1). In older books where cordless phones were the hot new thing this choice isn't an option, but if your story is set in a world with security cameras, streaming video, and phones smarter than their users, you have to take that into account.
There are other situations where this clause comes into effect, of course. The key is to look at every decision your characters make, and every twist your plot takes, and ask yourself why. For example, if your by-the-book police inspector has a hunch, and it's one he could very easily confirm through a little bit of investigative legwork, why would he start kicking in doors and pulling his gun without confirming the facts?
The reason is because it's exciting, but that excitement comes at the cost of the suspension of disbelief.