When I was a boy, I read a comic book about which I remember only one scene: the protagonists are being menaced by a bad guy with a gun. They get backed into a corner (literally, if memory serves, not figuratively), and just when it is obvious that there is no way they could possibly escape, the bad guy bursts into flames right before their eyes. One protagonist turns to the other and says, "Spontaneous human combustion: what a stroke of luck!"
This is an extreme case of a storytelling offense known as deus ex machina--literally, the god out of the machinery. The term derives from the Roman theater; as Roman theatergoers' appetite grew for novelty and plot twists (not to mention mistaken identities and twins separated at birth), the plots of Roman plays grew increasingly complex. In fact, they sometimes grew so convoluted that the playwrights gave up on actually resolving the complications in a narratively believable way. Instead, they would write a scene in which a god would appear and resolve all the characters' problems with the wave of a wand. That way, everybody could get home at a reasonable hour. The actor playing the god would often be lowered on a rope from machinery installed in the rafters for this purpose. Hence the phrase, the god out of the machinery, deus ex machina.
While many of us believe that there actually is a God who is fully able to reach out of the machinery of the universe to resolve problems of human making--a God who often does just that (and, indeed, who made the machinery)--few of us are interested in stories in which a human writer invents problems which he then resolves by inventing a divine intervention, or a happy coincidence or a timely case of spontaneous human combustion.
The deus ex machina highlights a tension that exists in almost all storytelling, both fiction and non-fiction. When we tell stories, we are balancing goals that are often at odds with one another. On the one hand, the storyteller is always trying to depict events that feel true to the way things actually happen in the world God made. On the other hand, the writer has other goals as well: he wants to communicate information that the reader needs to know in order to make sense of the story--information about characters and their relations to one another, information about setting, perhaps information about events that have led up to the events of the story at hand. He wants to create tension, then resolve that tension.* It isn't always easy to harmonize these goals with the goal of writing stories that feel true to real life.
The deus ex machina is one of the more spectacular failures to harmonize these goals. But there are lots of ways to get this wrong. No doubt you've read dialogue in which the characters seem to be talking to the audience rather than to one another.
Cindy, I've been your stepmother for ten years now, and I love you almost as much as I love my natural-born children--Paul, age 20, and Baby Brittany, your half-sister and the apple of your father's eye. But I don't think you have ever made me this angry before...except possibly the time you refused to be the flower girl at my wedding to your father.
It is always a problem when the writer's motives are easier to understand than the characters' motives.
A few weeks ago, one of my online students submitted a (non-fiction) story in which two co-workers have a conversation that gives the reader everything she needs to know about the two characters' relationship, teeing up the surprising gesture that is the climax of the story. The only problem was that the characters' dialogue was just a little too informative. I told the writer that the dialogue seemed packaged. (I might have said that the dialogue seemed like something written rather than something two people would say to one another in the world God made.)
The writer wrote back,
You're right that the story is 'packaged' in the sense that I chose to combine elements of different conversations and events that happened at different times into a single narrative; I was attempting to use those to establish the nature of the characters' relationships with each other, to give some context in which to understand the gesture at the end. But the events and conversations described really happened: they really said (more or less) those words.
Let me say two things in this writer's defense. First, it is the writer's prerogative to compress events and combine bits of different conversations into a single conversation, as long as he is not making any claims to reportorial accuracy (which this writer wasn't). And second, "to establish the nature of the characters' relationships with each other" and "to give some context in which to understand the gesture at the end" are both perfectly legitimate goals.
Having said that, I think it is dangerous for a writer to prioritize any goal over the goal of depicting a scene that feels like something that could happen in the world in which we live and move and have our being. Compressing events and combining conversations are fine as long as as the end result is a scene that feels like real life. Establishing characters' relationships and providing context are legitimate goals, but if you don't achieve the goal of writing scenes that could have happened in the world God made, you can't achieve any other goals in your storytelling.
As Flannery O'Connor said, do whatever you can get away with...but nobody ever got away with much.