If you don't believe supporting roles are key to great stories, let me ask you something:
What is Star Wars without Vader?
What is Pirates of the Caribbean without Jack Sparrow?
Would Dorothy be so interesting if not for her companions?
How about Mr. Potter? The most interesting character isn't Potter or even his friends -- it's that big dude who befriends spiders and unicorns, Hagrid.
What about Lord of the Rings, you ask? It would hardly be LOTR if not for Gandolf and Gollum.
You get what I'm saying, don't you? Every story has one character -- and often multiple characters -- who pull the story along. These characters are the interesting ones, the memorable characters who draw us into the story and keep us reading along.
Here's the ruse, though: Often they are ~supporting roles~!
The Main Character (MC) has to worry about the plot, the conflict, the one-two-three of the chapterizing, manage the wordcount, and drive from beginning to end. This can be agonizing, boring, painful, not to mention rote. Your MC is all business and no play.
So in stomp these supporting players to tear up the stage. They tickle the plot and move it forward, but really, most of them can be removed or replaced or edited down to minor bits. It's only the MC who is irreplaceable. If that's not true, you have the wrong MC!
For instance, let's look at Gollum in LOTR. He serves the MC as a guide, a foil, and an antagonist, and serves the author to enhances the backstory. None of these things were ~necessary~ to the plot. Frodo could have found his own trail into Mordor and never met Gollum. Heck, he didn't even need his good buddy Sam to tag along, did he?
But the trip itself was awfully boring. One-two-three. Describe the mountains. Four-five-six. Kill a spider. Seven-eight-nine. Burn a ring, ten you're done, let's go home.
Instead of moving linearly, though, JRRT introduces Gollum, a sidebar character who is, above all things, interesting and memorable. The story, if not the plot, just got better.
I could go on, but I think you get the picture. Along with your MC -- who should be interesting in their own rite, but may not be the ~most~ interesting character in the book -- you need to include sidebar characters who not only enhance the plot, but also draw the reader into the story.
Make these sidebar guys and gals and monsters interesting. Let them steal the show. Let them trip up and push along your MC.
Let them enrich your world and your story, so your MC can trudge along the plotted path and get done with it already.
Oh, and what does the monkey flipping you the bird have to do with this post? Absolutely nothing. He's simply there to demonstrate my point: He's interesting. My friend took this picture recently.
What are your thoughts? How important are supporting roles in the reader's experience?