Back in the day, before I became a responsible washing-machine-fixing parent type, I travelled round Egypt with a boyfriend. This meant many interminable, uncomfortable bus journeys. On one of them, we were sat next to a pair of American backpackers. My boyfriend and the male backpacker talked across the aisle, while the girlfriends in the window seats read books.

Two months later, my boyfriend emailed me at work:

Remember those backpackers from the coach to Abu Simbel? I told them to look me up when they got to London and we arranged to do something tonight, but I forgot I have to work this evening. So I’ve given them your address and they’ll be there at 7.

It’s OK, I dumped him.

Anyway, they turned up at my tiny flat which I shared with two friends, neither of whom was excited about having a couple of randoms interrupt our planned viewing of the World Cup…

And it was brilliant. The conversation went from stilted hellos to hilarity within minutes. We all talked non-stop. They moved from ‘isn’t soccer a girl’s game?’ to screaming and leaping with excitement at every goal, the god Phoebus Apollo was playing for England in his incarnation as the young David Beckham, we didn’t run out of beer though I have no idea how. They stayed till the last tube, and after they left, my flatmates and I had this conversation:

Flatmate 1: You know something weird? We’re in their travelling story now. “Remember the time we met that English guy in Egypt and ended up watching the World Cup and getting hammered with total strangers in London?” They won’t bore on about Big Ben when they mention visiting London, they’ll be talking about hanging out with us.

Flatmate 2: You mean…we’re like…their supporting cast?

Flatmate 1: … Oh my God. I’m a minor character.

Me: Do we get our names in the opening credits? “Special Guest Star  – KJ Charles?”

Flatmate 1: You wish. Half way down the end credits. *And* you’ll only be listed as “Girl in Flat”.

The thing is, we’re all supporting cast in each other’s stories. George Eliot in Middlemarch puts it significantly better than my flatmate:

[A mirror] will be minutely and multitudinously scratched in all directions; but place now against it a lighted candle as a centre of illumination, and lo! the scratches will seem to arrange themselves in a fine series of concentric circles round that little sun. It is demonstrable that the scratches are going everywhere impartially, and it is only your candle which produces the flattering illusion of a concentric arrangement.

The relevance of all this to writing? (Yes! There is relevance!) Supporting cast are not supporting to themselves. They may have a place in the book purely because of their interactions with the main characters, but that can’t be their motivation. If the doctor makes sexist remarks purely so the heroine can show off her snappy comebacks, he’ll be implausible. If the policeman pursues the heroes because the plot requires a secondary antagonist, he’s not a character, he’s one of those annoying Hero’s Journey archetype things.

So move the candle. Before writing the pursuing policeman, try imagining the story from his point of view, with everyone else as his supporting cast. What’s he actually after? How much does he care about arresting our heroes, as opposed to just improving his arrest record somehow? If he’s going to doggedly pursue your heroes through three books, why? Does he do that with everyone he wants to arrest? Why would he have this obsession if not because he’s a secondary character in their story?

That doesn’t mean that the reader needs (or wants) to hear about the motivation and backstory of every character. (Regular visitors may recall that I’m big on the author knowing things that she doesn’t tell the readers.) I’m currently editing a novel in which the author has given all the minor characters a ton of backstory (from the lawyer’s irrelevant childhood on an army base to the father’s favourite film) and I’m going Red Pen Crazy because all that stuff is bringing the book to a grinding halt.

But when that’s all gone, it will still be detectable that these characters have a hinterland and a personality, not just a plot role. Because the author knows who they are and what they want, her knowledge informs the writing in a thousand tiny, subtle ways. They have individual, plausible reactions. The main characters interact with them as people, not as information providers or Threshold Guardians. They have their own needs that aren’t neatly aligned to the main characters’ story. And that makes the book richer, probably better plotted, and more true.