When Isaac Newton first came up with the theory of gravity, he presented what seemed like a simple problem.  If you have three bodies orbiting around each another, how do you come up with an equation that describes how they move?  Two hundred years or so later, mathematicians came up with the answer – you can’t.  It’s easy enough to describe two bodies orbiting around one another – the Earth and the Sun, say – but once you throw in more bodies – the moon, for instance, or the other planets – then you can’t really predict what will happen.  The extra bodies throw the system into chaos.  This is why you’re always seeing news reports about celestial events that won’t happen again until some seemingly arbitrary time in the future.

When you’re writing a novel, you naturally center on your hero or heroine, tracking his or her fears and desires to your big climax.  That’s where your story lies.  With a mystery, you’ve also got to consider the perspective of a second character, the murderer.  Romances, too, involve two characters whose lives circle around one another, as do thrillers that include a well-developed villain.  All these stories tell how these two characters orbit one another and come together at the end.

But most stories have more than two characters, which means you’ve also got these other people with their own motives and desires circling around your main characters.   How you handle these other characters – how much independence and individuality you give them – can change how much your fictional world feels like real life.

The obvious mistake — and it’s easier to make than you might imagine — is to simplify the other characters to the point where they become props.  They don’t really have internal lives or backstories of their own, they’re just there to play a specific role in the story – feeding your readers critical information, or causing some complication at a critical point.  Granted, you don’t have to delve deeply into the motives and history of the delivery guy or the cop who pulls the heroine over that one time.  Their characters don’t have to amount to more than a couple of telling, idiosyncratic details.

But when your supporting actors – outside the orbit of your one or two main characters, but still players – are nothing more than props, your story is going to feel less authentic.  After all, everyone but a complete narcissist knows that other people are living lives as unique and complicated as their own.  So if your hero has a friend whose only purpose is to provide comic relief, or if your heroine has an old lover who is only there to play the rival, your readers simply aren’t going to believe in your world.

After Agatha Christie’s first marriage — to a handsome bounder — fell apart, the villain of many of her mysteries was the handsome young man with the evil heart, who was often little more than a plot device. These mysteries are often entertaining as puzzles – she was still Agatha Christie, after all — but they are not her most memorable works, in part because her villains became stock characters with no individuality.

So how do you bring your secondary characters to more convincing life?  One exercise that can help is to rewrite key scenes from the point of view of one of the more minor characters.  Even though those scenes won’t wind up in the final draft, they force you into the heads of your supporting actors so you can see what your major plot developments look like to them.  This makes it easier to give them their own stories. And once your minor characters come to life, what they do might surprise you.

It also might throw your story into chaos.  If all of your characters are living their own independent lives, there’s no reason those lives should dovetail together to form a single, well-paced story with nicely-timed twists and occasional surprises.  This may be why writers who follow their characters rather than outlining their stories in advance occasionally wind up with an incoherent mess.  So how do you hit the right balance between making your minor characters people and making them behave themselves so you can write your story?

Subplots are one way to do it.  Humanize your secondary characters by giving them their own, independent plot threads not really connected to the main plot.  This lets you people your novel with living human beings without overly complicating your story.  Blood Never Dies, one of Cynthia Harrod-Eagles’ Bill Slider mysteries, lets us watch not only the search for the victim’s identity and pursuit of the killer, but the development of McLaren’s romance with a young lady who has him eating more healthy meals and losing weight (they break up in the end and he goes back to curry and fry-ups).  But subplots can still feel artificial, especially if they have little or nothing to do with the main plot.  In the real world, our lives are more intertwined than that.

Another way to fold your secondary characters into your story is to have your climax – the moment when the hero’s story resolves – also mean something to the minor characters.  If a single event wraps up everyone’s stories at once, then what came before is going to feel more coherent.  And you can make the characters’ various stories feel more independent if the climax means something slightly different to all of them.  When the couple in your romance finally get together, the rival who loses out may be freed from a nostalgic dream of past romance and be able to move on.

You can also take advantage of the fact that, if all these characters are in the same story, they’re probably connected in some way.  The lives of people circling in the same orbit often do dovetail together, with each character’s story affecting all the others.  What you’re looking for is the right balance between interconnectedness and independence – minor characters who are unique individuals but not so much that they undermine the structure of the story.

Alice Hoffman’s Practical Magic [spoilers ahead] is a good example of how to do it well.  The story centers around two orphaned sisters, Sally and Gillian, raised by a pair of spooky maiden aunts who are (perhaps justly) considered witches in their hometown. The town’s unease with the aunts leaves the girls socially isolated, and the aunts themselves are often cold and demanding.  Eventually, the two sisters escape in different ways.  Sally marries and settles into a conventional, responsible, boringly ordinary life, remaining single after her husband dies.  Gillian takes off at 18 and spends the next years moving from town to town and man to man.  Their lives collide once again when Gillian shows up in Sally’s driveway with the body of her latest man, Jimmy, in the car.  Together, they bury Jimmy in the garden, and the rest of the story centers on how the sisters reconcile with each other while they deal with the fallout from Jimmy’s death, including being haunted by his ghost.

But there’s also Gary, the investigator looking into Jimmy’s crimes, whose arrival puts extra pressure on Gillian and Sally, but who has his own story of loss and isolation that keeps him from simply playing the role of “the relentless investigator.”  Eventually he finds love with Sally, and she with him, which brings both of their stories to a satisfying conclusion.

There’s Sally’s two teenage daughters, Kylie and Antonia, whose own peaceful lives and budding loves are shaken up by Gillian’s arrival and the upheaval that follows.  When they are invited to help Gillian and Sally deal with Jimmy at the end, it’s an important part of their coming of age, of being accepted as adults by their mother and themselves.  The ending wraps up their stories as well, but it means something different to them than it means to Gillian and Sally.

Then there are the aunts.  At the beginning of the story, they are simply a force of nature, two brooding maiden ladies in black presiding over and dominating Gillian and Sally’s lives. Readers only learn their names – Francis and Jet – near the end, after Gillian and Sally turn to them for help in dealing with Jimmy’s ghost.  And while brewing potions in Sally’s pasta pot to settle Jimmy’s spirit, they finally have the chance to express the love they never really showed during the girls’ childhoods – as they’d learned they needed to do by watching Oprah.

One reason Hoffman’s book feels so real, despite the magic woven through it, is that all of her characters circle around one another, each on his or her own trajectory, the way we live in real life.  Yet all of their lives weave together into a single, well-paced, satisfying story.

Finally, if the music of the spheres doesn’t work for you, here’s another way to look at the question.