The Best Friend / Confident
Surely this article has to start with a hat tip to the best friend / confident. Where my Horatios at? I gotta stop writing these drunk. The best friend / confident is a great alternative to the soliloquy. We know what Hamlet is thinking because he tells Horatio all about it. We know what Ladybird, and Juno, and so many other characters are thinking because they call up their friend and tell them! This character is a great way to get exposition into the world!
Give them an Arc of their Own
Instead of just making them an addition to the protagonist’s journey, consider giving them a meaningful journey of their own. What is an arc exactly? Hard to explain quickly, but it’s basically a journey from one state to another. From one set of beliefs to another. It’s what the protagonist learns, or the change they go through, due to the events of the tale. What they learn is often the point, or moral, of the story. So consider giving your supporting character their own arc. A classic example would be Wizard of Oz, where Tin Man gets a … brain is it? And Lion gets a … it’s gotta be heart right? And the Wicked Witch gets a bucket of water.
Have that Arc Contribute to the Protagonist’s Arc
It’s not a bad idea to make your supporting character’s arc contribute to the overall arc of your protagonist in some way. This is a bit tricky and to be honest it’s still something I’m working out myself, but I’ve been told that by a very reputable source, so I’ll let you think about it and report back if I ever figure it out.
Have them Provide what the Story is Missing
So you’ve written a tragedy? Get some funny characters in their like Rosencrantz and Guildenstern. You’re writing a comedy? Get a very sad best friend in there who is on the brink of suicide to add a little emotional heft. Have fun.
Use them to enrich the Variety of Characters and Issues Explored
If this is a story about kings, get a poor porter in there. If it’s a story about poor people, get a rich uncle in, or a boss. The point here is to flesh out the philosophical and ethical density of your work by portraying a wider cross section of society. If you’re some sort of wacky genius like Boon Joon-ho, director of Parasite, then you can blend everyone in and leave the audience not entirely certain who is a main character, and who is supporting, but talk about commenting on societal inequality through portraying a broad range of characters!