For me, a Sub-Plot is anything that takes the characters away from the main thread of the story on a merry little jaunt of their own, and then weaves its way back towards the main thread. They’re the plots that as a Gardener writer, there is a pause between coming up with the idea and typing it down, during which I think do I really want to try this?
(Spoiler alert, if it’s the first draft, then yes give it a go).
Sub-Plots are your opportunity to play around with your characters, and take your reader somewhere new. It’s a great opportunity to show elements of your world that might not otherwise be mentioned (e.g. visiting a festival or a local historical site), and it also allows your characters to stand on their own – especially if your sub-Plots have your main band of characters splitting up for a period of time.
The other advantage with sub-Plots is you can build really unique POVs for each character. Rather than having your characters stay together as one group, you can send them off on different “side-quests” so that they can experience different events by themselves. This is particularly useful if you have an ensemble cast and you are trying to avoid re-hashing conversations with the same characters. Split ‘em up, put them somewhere dangerous, and see what new life can be breathed into your story!
Sub-Plots can also be used really effectively to play a game of smoke-and-mirrors with your readers. Your readers will expect that the sub-Plots will tie in with one another (as is the expectation of any story), so they’re going to start guessing how it fits into the main plot. Then, when you reveal how it fits, you give the reader a huge payoff!
Do you use sub-plots in your current project? If not, maybe consider how you can incorporate them to the advantage of the main story!