“Contradictions do not exist. Whenever you think you are facing a contradiction, check your premises. You will find that one of them is wrong.” -Ayn Rand
Ayn Rand, author of Fountainhead and Atlas Shrugged among other dense pieces of Objectivist literature was known for these sorts of sweeping statements on moral philosophy. While many would argue how many of them were true, this one is of particular importance to storytellers.
One of the greatest dangers for a storyteller is internal inconsistencies. People have a deeply seated sense for reality. You could have a world of Dwarves, Elves and Vampires, it doesn’t really matter as long as these supernatural creatures follow the rules you set out for them. They need to behave according to the premises you’ve established. When they don’t, it breaks the contract between you and your audience and you can quickly lose them in the mire of incredulity.
Check your stories for inconsistencies, and take the time to spell out your premises. If your hero is a pacifist, he probably shouldn’t get into a bar fight in the second act (unless there is a spectacularly good reason). If you live in society where farm land has been obliterated by disease, having your characters enjoy a fresh bowl of pasta (unless you also have Star Trek-esque replicators) doesn’t make sense.
Although most people loved the film 28 Days Later, one of the criticisms against it is that it’s not a real “zombie movie.” The reason is that zombies don’t run, and zombies are dead. These are two rules that the movie broke rather spectacularly. Even here though, the monsters of 28 Day’s Later follow the rules internal to the storyline.