Every character in every book has their own story, and each character is the hero of his or her own story. This piece of writing wisdom has been around for at least as long as the novel has, but too often, writers don’t think about the implications with regard to whatever story they are writing.
What it means is that every story is composed of multiple, overlapping stories, not all of which can (or should) be told in full. Sometimes, this is because the main plotline is important enough and strong enough that it will still be the “main story” for most of the other characters during that time period. If you have six central characters who are members of a bomb squad, and the main story revolves around locating and destroying a bomb before the World Series starts, they may each have a slightly different view of what is going on and why they are doing it, but the central story, for those six hours, is going to be that they have a bomb to locate and disarm. There’s so much overlap that giving each of the six a viewpoint and a storyline is going to be redundant (unless the author finds some other aspect of their personal story to focus on, which I’ll get to in a minute).
Other times, though, the individual character’s story doesn’t have enough overlap with the central plotline. The cab driver who brings Greg to the stadium to hook up with the rest of the bomb squad has a story…but the cabbie’s story revolves around trying to get his sister (who has cancer) into an experimental treatment program. It could be an intensely emotional and moving story, but it has virtually zero overlap with the bomb story that the author is telling. It’s pretty easy to see that it doesn’t belong in the book.
Or at least, it’s easy from my description. For the author writing the story, it sometimes isn’t so easy. What usually happens is, the author needs or wants an outside view of Greg, so he throws in the cab ride scene from the cabbie’s viewpoint. But once the author is looking through the cabbie’s eyes, the story looks different. First off, the cabbie is usually a lot more interesting from the inside. Second, his story feels suddenly much more important (because, to him, it is). The author’s natural instinct is to do more with both these things, and from there, one of three things tends to happen: First, the author gets totally distracted and goes haring off after the cabbie’s story, forgetting entirely about the bomb story; second, the author has gotten so interested in (and put so much effort into) the cabbie’s story that he/she can’t bear to just let it go, so the cabbie’s story becomes a subplot; or third, the author starts looking for ways to link the cabbie more directly into the bomb story.
Each of these alternatives can work brilliantly, and each can fail miserably. An author who starts off writing an action-adventure bomb novel but who ends up with a moving story about the cabbie and his family is fine; an author who starts with the bomb story, then writes about the cabbie for a while, then gets distracted by the ER tech who shows up when the sister collapses, then gets interested in the ER tech’s brother who teaches school in Boston and is just heading home after a visit, then jumps to the flight attendant on the brother’s plane back to Boston…well, it may be possible for somebody to come up with a coherent novel out of all that, but it’s more likely that the author will end up with a bunch of incidents, fragments, or not-really-related short stories. Making the cabbie’s story a subplot can also work, if there are thematic ties to the bomb story, or it can be really awkward and self-indulgent. Linking the cabbie into the bomb plot can feel too artificial and/or coincidental, or it can become a brilliant plot twist. It’s all in the execution.
Between the extremes – the main characters whose stories are currently too close to the central plotline and the bit players whose stories have almost nothing to do with the central plotline – are the characters whose stories overlap the central bomb plotline, but from a totally different angle: the stadium manager who is responsible for the safety of the fans, the police chief who has to make the decision whether to call off the game, the mercenary who helped set up the bomb but who is having second thoughts, the players and team managers, various ordinary people who have tickets to the game and who the reader knows are potential victims. For each of these people, their personal main story may be “about” something entirely different (a lost love, a chance for a comeback, making up for some previous mistake or righting some major wrong that’s irrelevant to the bomb threat), but for these few hours, the bomb story is a minor thread in their story – a subplot – which makes them possible subplots for the bomb story.
Choosing among all these possible story threads and subplots requires one of two things: either a clear focus and commitment that one can use to decide, or a highly active intuition that allows one to make those choices on pure faith. Focus and commitment can be to a central plot thread, a theme, a structure – anything that the writer is clear on that allows them to say “I will include the hot-dog vendor, because his story ties into or reflects X, but I won’t include the bat boy, because his story doesn’t.”