I know I've talked about this before, but I strongly believe that in any good story, it's the characters doing the work, not the narrator. Let's make a distinction here: narrator and writer are two different entities. While a writer's voice may be unique from other writers, and while their voice may be similar in all of their books, the writer is a real person and the narrator is another character in the story. A character, I might add, that should be all but invisible. Except in first-person POV. Of course there are a few exceptions where the narrator inserts his own voice into the narrative, breaking the fourth wall to talk to the reader, but that's the exception, not the rule.
Anyway, on to my actual topic: characters are what make a story. If those characters are flat, or if they simply sit around waiting for the narrator to tell the reader what they did, then the story is boring. As far as the writer is concerned, those characters that aren't doing something on the page should at least be doing something in the writer's mind. An easy example comes from mystery novels: just because we see the protagonist hunting down clues to prove that the murderer is the man we believe him to be, it doesn't mean that the antagonist is sitting around waiting for us to look at him. The protagonist is looking for clues on the page, but the murderer is across town, mucking around with another, possibly unrelated, crime-scene in an attempt to frame the protagonist's cousin in two separate crimes.
If planning an example out like that sounds like it takes a lot of work, that's because it does! I've tried to write a mystery in the past, and it's not easy. I hope to write one soon, but I do a lot of that kind of work in all of my stories. I may not know what each and every character is doing while they're not on the page, but if they're changing the story in any significant manner, I plan it out before I write it.
I even use that writing technique when I'm running a role-playing game. Just because my players don't see an NPC, it doesn't mean that a character isn't running around ready to send a plant-monster at them when they move on to the next scene. (And for those of my players who read this blog; that was not a clue.)
This is why characters must be thought out well. We need to know that they have depth because as readers, if we see that your characters do stuff (like shuffle their feet in a corner of the bar, drink coffee while the detective interrogates their sister, rolls their eyes at a corny joke, etc.) we'll assume that they have a life when they're not on the page. We don't always need to know what you intended them to do, but if you don't fill in those behind-the-scenes blanks, we'll do that for you...as long as the characters aren't set-pieces. If you plan out off-page actions, it will come through in your writing, and we'll love what we read.