Stories are the collection of smaller pieces that build into something more significant. This statement isn't profound or even new, but it is something that many of us forget a story can be when we stick with the main characters only.
I was recently watching Nerdwriter on YouTube, and he did an episode on How Not To Adapt A Movie. I highly recommend watching it:
I agree with many of the points made in this video, but the part that got me the most was the use of minor characters and how they broaden the fictional world you are creating. As writers, it is easy to hitch ourselves to the main character's wagon and ride along for their story alone, but using side characters to help make the world feel a bit more real is something that I love and have forgotten to use in my body of work.
After watching this video, I will be fixing this problem, as it is something that I feel should be utilized more in the shared narrative consciousness of our society. If there is one thing that is missing in our real world it's that everyone has a story to tell and those stories matter; and if our fiction had more of those vignettes in them then maybe we'd actually give a shit about the folks around us rather than the 'heroes' we're told to hate or love.