What do people do when you're not watching? Do they exist? Does it matter? Are they these creepy dead things that only come to life when you're playing with them? (Dolls might be the word I'm looking for.)
I have friends who tell me that they worry that people are dead, when they are not interacting with them. I have friends who assume that all the fun things happen when they're not around. I personally tend to assume everyone is in something of a state of suspended animation, with occasional worry that they're actually dead, or suspicion that they're having more fun without me.
This is a heady philosophical question, but I'd like to apply it instead to stories. When we write, there are a few tendencies that can pop out. One is to blindly tell a story and make everything fit into it, regardless of characters', well, characters. Sometimes it doesn't make sense that a person like that character would do that thing, though it helps the plot. Sometimes I get the feeling that the character has in fact been in suspended animation in between needing her to be on the page. Sometimes I suspect that were I to ask the character what he did over the weekend, all I'd get is a blank stare.
Other times, the tendency is to overdescribe all the occurrences. It's like what would happen if purple prose answered all questions with where and when, as well as how and pretty. Do you ever have those moments where you look back over a scene you just wrote and think "erm, did I need to write everything that happened to her that day? Because um, it turns out, nothing happened. But she drank some tea! And read the newspaper. In which there were articles that I described faithfully. Then she went to the loo! Then, then, then." Forgetting how to fade to black and jump timelines would be sad in real life, but is tragic indeed in writing. What do you mean you don't have those abilities in real life? What do you mean those abilities are only brought about by excessive drinking?
One of the things that fandom taught me was to think about the in-between spaces in a story. It taught me wonderfully how to develop a character in the tiniest ways - precisely because fandom would then take those tiny signals and enlarge them hugely - and what can be left unspoken. It made me think about the character's introspection and deepest murkiest (or chirpy) thoughts that I as an author need to know about, lo tho they may never achieve the page. It's ok, anxious author! The fanficcers will spell it out for you! I promise.
So it simultaneously made me learn how to flesh out a character, and how to do it in subtle ways and not actually put it all on paper. Sometimes it's enough to know it all in your head as you're writing; it'll shine through as the characters feel more alive and realistic.
And when all else fails, the fanficcers will make all of your characters have all that sex you haven't been letting them have.