Do you start out knowing all of the subplots? Or do they tumble and bump into each other along the way? Are there certain ways you like to develop subplots? Or do they just come to you? Are they villain driven?
As I believe I’ve said before, I don’t recommend my method. I never know what the hell I’m doing in the beginning. I just write. Characters show up. Some of them are interesting enough they develop their own plot lines. Some of those I have to put the kibosh on because they’re cluttering up the story (good-bye, Mort). Some of them echo the main plot or act as a foil to the main plot, and they deepen what’s happening in the story as a whole, so I keep them (hello, Max and Button). So for the first discovery draft, I just let them happen. After that, as always, I analyze. And to analyze I go back to basic plot structure.
Subplots, like main plots, are protagonist driven and antagonist shaped. (To your question “Are they villain driven?”, if the antagonist drives the plot, he or she is not the antagonist, he or she is the protagonist.) But subplots have another function: they act as echos or foils to the main plot, deepening it by either showing another aspect of its thesis or showing a contrast to it.
Example: In Faking It, Tilda and Davy are both trying to go straight, but in order to keep Tilda out of jail, they both have to go back to conning people including each other. That screws with their relationship, and they can’t straighten it out until they tell each other the truth. Meanwhile, Tilda’s sister Eve and Davy’s friend Simon have a great sex life because they’re lying in their teeth to each other; their fantasies are what fuel their passion. So when Eve finally tells Simon the truth, he ends their relationship, his fantasy destroyed, and Eve is furious with him when she learns his truth, her fantasy destroyed. That Eve/Simon subplot is a contrast to the Tilda/Davy main plot because the Tilda/Davy relationship is built through working together and understanding each other and finally trusting each other enough to tell the truth; whereas the Eve/Simon relationship can only last as long as they lie to each other. The Eve/Simon subplot is necessary to the story as a foil for the main plot. (Tilda’s antagonist is Clea; Eve’s antagonist is Simon.)
So I look at each of the subplots that are emerging and ask, “How is this going to make the book sharper, deeper, better? How does it relate to the main plot? How is it necessary?” If it isn’t, I get rid of the subplot. Right now I have too many romance subplots in Nita, everybody in the story falls in love, and that’s ridiculous.
So I look at how the subplots echo or contrast with the main plot. The main romance plot is about two people who are completely alone: Nita, a one of a kind supernatural being in a world full of humans and Nick, a dead human living in a world full of live demons. A contrast to that would be a relationship between two insiders surrounded by a support group, like the youngest member of a long line of demon killers and a fixer who’s the latest in a long line of demons; Button and Max bond because they’re both action-oriented, determined to clear obstacles from the paths of those they protect while dealing with the weight of their families’ histories. The fact that Nita, Nick, Button, and Max are all basically fixers helps bond them into a team, but the contrast between the two romances heightens how much more difficult things are for Nita and Nick. Button and Max are used to connecting with people so they fall for each other pretty easily. Nita and Nick are used to being alone, so they struggle.
The other romances don’t resonate as well. Jeo and Daphne fall for each other because they’re young and attractive and that’s about as far as I got there; I wanted Daphne in there so that Jeo would turn down Nick’s offer to make him his heir. Rab and Dom have even less motivation; they’re both information junkies with big hearts and big brains, but I don’t do anything with that, either. The one thing both romances have going for them thematically is that they’re inter-species human/demon, and a big part of this book is accepting who you are and who others are. They’re also, by accident not design, mixed race romances, which plays into that theme. I really don’t have the page real estate to devote to developing those subplots, which means that they could go and the book wouldn’t be hugely damaged in the way that losing the Button/Max subplot would hobble the plot.
It’s important to begin subplots after the main plot begins because you don’t want the reader mistaking the subplot protagonist for the story protagonist and bonding with him or her (and because the main plot doesn’t being until the protagonist steps on stage because the story belongs to her/him), and they end before the main plot ends because you don’t want them dragging out the end of the book. They should be shorter than the main plot because you don’t want them to be more compelling than the main plot and much less complex because they can’t claim too much page real estate without threatening that main plot.
Basically, subplots are developed and constructed just like main plots, except instead of being The Story, they’re in service to The Story.