Many writers will tell you that, in building a story, every scene has to push the narrative forward. What does this mean?
Most of the scenes have to build tension and anticipation.
Tension is stress, and you want that stress relieved. Anticipation is anxiety, that feeling that makes you want to confirm a hunch.
1) To build tension, introduce Challenges. They are meant to prevent your character from moving closer to achieving his goal. Challenges come in the form of conflict--whether physical (a confrontation), mental (a puzzle), or emotional (doubt, anger, despair, devastation). When you introduce a Challenge in your story, you should give the reader an impression that there is a chance of failure. In Arkham Asylum (by Grant Morrison and Dave McKean), Batman is subjected to numerous challenges, mostly mental and emotional, as he tries to keep his sanity intact with each encounter with the occupants of the asylum.
2) To build anticipation, introduce Reveals, or relevant knowledge. These are what your character learns through his own investigation, through revelations from other characters, or by sheer luck. Grant Morrison successfully used Reveals in his All New X-Men story arc where Emma Frost was murdered, her diamond form shattered into pieces. As Bishop investigates, the readers learn that a number of characters had a clear motive for committing the murder, but nothing's conclusive. Reveals, once they start piling, make the reader try to piece them together. Anticipation is built because the reader becomes a participant in the solving of the mystery.
A Reveal, however, is not limited to what your main character discovers or finds out. You can also introduce Reveals that the reader finds out ahead of your character. A classic example is the "killer behind the door" scene. If your hero approaches a door, and the reader knows that the killer is behind it, the anticipation is far greater than if the reader has no idea what's behind the door. Anticipation is built when the reader thinks in advance about what might happen next, and stays around to confirm it.
Reveals aren't also all about the main story. You can also have Reveals that tell a bit more about your character, or other characters. The more humanlike your characters are, the more you're giving readers a reason to connect and empathize with them.
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The middle part of your story is where you dish out Reveals and Challenges in regular doses. If you don't bring in a Reveal or a Challenge over a number of scenes, your story dips. You're giving your readers little reason to stick around. So, ideally, every scene must have at least one Reveal, or one Challenge, or both.
There are big Reveals and Challenges and there are small ones. In feature films, as well as graphic novels that don't have act or chapter breaks, there are usually three or four big Challenges spaced equally apart, with small ones in between. (To clarify, a "big" Challenge doesn't always mean "explosive" or "highly kinetic." A big Challenge could be an internal one, such as a hero witnessing the death of his parents.)
In comic books, big Reveals and/or the start of big Challenges are usually delivered at the end of an issue, because we want the readers to keep on buying.
So if you're planning to have a six-issue limited series for your story, think about what the big and small Reveals and Challenges you would need each issue to contain. If your story doesn't have a lot of big Challenges or big Reveals, six issues might be too much.
NEXT: To demonstrate the idea of Challenges and Reveals, let's take a popular comics series apart and see what's going on inside. Breaking down "The Walking Dead Vol. 1: Days Gone By" is next.