“subplot – a secondary sequence of actions in a dramatic or narrative work, usually involving characters of lesser importance (and often of lower social status).” – The Concise Oxford Dictionary of Literary Terms
Subplots are one of the largest and most obvious differences between short stories and novels. There is rarely enough room in a short story, or even a novelette, for even one subplot, let alone several, while a novel often needs multiple subplots just to keep from feeling flat. Consequently, subplots are one of the things that a lot of short story writers have trouble with when they start writing longer work.
The first mistake a lot of these folks make is in looking at subplots as something they have to graft onto their main storyline, consciously and deliberately. They treat their subplots as a bunch of independent semi-stories that have to be brought together like different threads being tied up in a knot, all of which remain discrete and separate strands.
But subplots are more like the leaves on a long-stemmed rose – they’re part of the same plant, and they feed the stem and the flower. They can illuminate different aspects of the main characters or the central plotline; reinforce or reinterpret the theme of the story; expand the scope of the world beyond the a narrowly focused storyline or put human faces and feelings on a broad, sweeping storyline; or provide the reader with insight and information that they otherwise wouldn’t get. Subplots are part of what makes novels rich and gives them more depth.
The second mistake a lot of folks make is in thinking that they have to know exactly what the reason is for the presence of each subplot, what purpose it serves, and what it adds to the story, before they’re allowed to put it in. “This looks fun and interesting” isn’t good enough for them.
Fiction is supposed to be fun and interesting. Oh, some writers do create their subplots very consciously and mechanically: “Let’s see, the main story is an action-adventure plot focussed on bringing back the magical doohicky from the End of the World. I want the hero to get married at the end, so I’ll need a romantic interest and a romantic subplot; stick that into chapters 3, 7, 11-12, 15, and of course the final resolution. And I think I want a little more emotional depth; better come up with something internal that he can search for — like courage or a brain — at the same time he’s hunting up the magical doohicky…” There’s nothing wrong with working that way, if it works for you. By the same token, though, “fun and interesting” is plenty enough justification for throwing something into the first draft. You can always take it out or make adjustments when it comes to the revisions phase.
And for most of the writers I know, subplots grow organically out of the events in the story, like the leaves growing out of the stem of the rose. That is, the writer doesn’t think “Ah, Chapter Three; time to introduce a subplot” and then deliberately insert a scene that adds a potential love interest or a political complication. What the writer does is more like, “Well, here’s my heroine, racing down Main Street to escape from the Evil Assassin Wizard, and she runs into this guy and fast talks him into hiding her…hey, he looks interesting, and they seem to be hitting it off. Maybe I’ll keep him around. I wonder what he does…oh, he works for the EPA. I bet they have regulations for magic pollution…” And next thing you know, the writer is juggling the ramifications of romance and politics as related to magic, in addition to the original plot.
I’m one of the latter sort. My subplots usually come about because I’m looking at everyone in the book, not just the main characters and the main storyline. Everybody has a story, including all the supporting characters. Some of their stories are necessarily tangled up with whatever my main storyline is, and their lives will be affected by whatever is going on (though, presumably, not so dramatically affected as my main characters, or else they’d be the main characters instead).
And I look at levels: there’s the action plot, the events that are happening; and then there’s the emotional plot, what those events mean to the main character. There’s what’s happening in the “big picture” (“Hey, we got a major riot down by the docks. Send out the City Guard and put a couple magicians on the alert…”) and there’s what’s happening up-close-and-personal (“Ye gods, that maniac just tried to brain me with a dead fish! Where do I find a safe spot until this riot is over?”). There’s what everyone thinks is going on (“Someone is trying to assassinate the King!”) and there’s what is really going on (it’s actually a convoluted plot to get the Royal Guard beefed up at the palace, so that somebody can steal a pair of seven-league boots from the hunting lodge more easily).
And I look at relationships: there’s what the heroine is doing under orders for her king; there’s all the stuff she’s been trying to prove to her father since forever; there’s her mixed feelings about her kid brother the lame wizard; there’s the bratty obnoxious witch she has to work with even though they hate each other; and so on.
A lot of this stuff just ends up being background, or not getting into the book at all. But sooner or later, some of it ties into whatever the main thrust of the book is, or I decide I really like one of the minor characters and want to see more of him/her, and presto, there’s a subplot. Though by that point, I don’t think of them as separate subplots. They’re just part of the story, like leaves on a rose.