Oh, the poor supporting character.
The best friend. The lab assistant. The cab driver. The sex gimp.
How shitty they must feel, you know? “Hey, we’re all blocks of flesh in the storytelling pyramid, meant to uphold the protagonist. Hey, pass me another bucket of plot, willya? I’m getting dry. What’s that? The antagonist stole the bucket of plot and pissed in it? We don’t have to… wait, we have to drink it? We have to drink it. … Goddamnit.”
Somewhere in here I’m envisioning a human centipede thing, except in pyramid shape and…
No. Nope. Hunh-hunh. Not going there.
You might think, hey, that’s the ideal usage for a support character. To support the characters, the plot, and the story. Maybe to uphold theme, too, or contribute to mood. And all of that is technically reasonable and not entirely untrue, but looking at it that way runs the risk of coloring your view of all characters as being no more than mere pulleys, gears and flywheels whose only purpose is to mechanize the plot you’ve created. (You ever see the ingredient mechanically-separated meat? It’s something like that, where you envision all the characters as avatars of plot diced up and separated out.)
Characters aren’t architecture, though.
Characters are architects.
Your protagonist and antagonist tend to be grand architects — they’re the ones making the big plans. They’re building — or demolishing — whole buildings. They are the demigods of this place. Creators. Destroyers. Sometimes each a bit of both.
But supporting characters are architects, too. They’re just architects of lesser scale. They work on individual floor designs. They’re hanging art. Moving light switches. Picking paint colors.