Have you ever started writing a story only to find your characters side-tracked by a morning traffic jam or sudden flood in the basement? Maybe these incidents develop into subplots that take on lives of their own, such as whether the traffic cop is actually worsening the snarl of cars or whether the plumbers secretly installed devices to ensure their services will be required later.
Admittedly, the subplots mentioned above are thin, but the point stands... subplots are a great way to increase your word count. Beyond that, subplots can help to expand the scope of your novel's world, and might tie back in to the main plot with a vital clue, uncommonly good timing, or serve as comedic relief.
For example, writing about the morning traffic jam allows you to explore the kind of car the main character drives, and how he or she drives it. Is she nervous about scratching it? Is he upset that the horsepower is being wasted in bumper-to-bumper traffic? The time the main character wastes on the road could be important later on in the story, as it could cause them to miss a meeting, lose their job, or avoid being at the office when that 7.4 earthquake hits.
The plumbers' subplot gives plenty of opportunity for description and emotions. What did the house or apartment look like before the flood of seemingly Biblical proportions happened? What does it look like now? Is the plumber sympathetic, businesslike, or annoyed at having to slosh through six inches of water and the main character's now-soggy collection of home decorating magazines?
The plumber subplot could tie back into the main story, because now the main character is familiar enough with the basement - courtesy of the hours spent down there, distrustful of the plumber - that she's able to outsmart the serial killer who invades her home in a later chapter.
Consider adding a subplot to your writing project, but try to make them ones that fit with the setting and characters. Cutting to chapters about King Arthur's court might not make sense in a modern-day urban fantasy, but you could try doing a subplot on Renaissance fairs or a comic book convention instead.
Let's try something a little different with the prompt today. I'll give a writing prompt and a subplot. Have fun, and keep writing!
Prompt: The sun was starting to dip below the horizon, and the guy at the stall sighed at me in annoyance as I worked to get the ring off my finger. I'd just wanted to try it on, but now the craft fair was closing up shop for the night. If I didn't make progress - and soon - I'd have to buy the ugly thing and drive to the nearest hospital or hardware store before I lost my finger...
Subplot: Jewelry thieves are plotting to steal from the craft fair, and nabbing this troublesome ring is one of their main goals.