sonder - roughly, the realization that everyone who passes around you, has a life as complex and intricate as your own, and that they are constantly living it even without you
That's some pretty heavy stuff. Our brains aren't great at something like that for long periods of time. It's easy to portray the person who cuts you off in traffic as being just an ass, ignoring that she might be in the midst of the worst day of her life. Or the kind and cute person at the grocery store actually is checking you out, while checking out your groceries. There's so much to process, trying to take all that in, trying to hold all of that in your head.
It feels to me like a fractal - there are infinite moments happening all the time, all over the place. There's a book that came out pretty recently, From a Different Point of View - the entire point of which was to, functionally, explore this idea in the Star Wars universe. You get stories from all sorts of bit players, each at the edges of the movies that started it all. There's a story by what is practically a sentient flea, telling about contributions to the success of the Rebel Alliance. From a droid in a single scene of the movie. From people who never even make it onto the screen, but absolutely had to have been there, just beyond the area covered by the camera. Reminding us that, yes, there's the rest of a universe out there.
Even with the scientific problems of Star Wars, the plot inconsistencies, the million and one other complaints that we could have about it, I think this is part of why the property lasts. We feel like people live in the world. When we learn about the bureaucrat that has to write up reports about Imperial Engagements, it clicks. It just fits.
Similarly, this was one of the complaints that some reviewers had about Mass Effect 2: that the universe felt more empty. It could have easily been a series of rooms that Shepard was herded through, rather than an entire galaxy.
I think that writers, like all of us, need to suffer from the feeling of sonder from time to time. To sit in it, to bask in it, to realize that the pieces that make us feel like we're living in our own world, in a world that is real and that if we scratch at the edges will just keep revealing more and more, and need to ensure that we're imparting that into our work. With the quirks that make characters not only more memorable, but also more real.
I've been editing my way through a novel that I wrote, in fits and starts, trying to ensure that I do this piece justice. That even though it isn't on the page, that each of those characters has a life, even if it is going home after a long and hard day of work, and just drinking a beer.