Impartial Observers
From KitLit.com
Sometimes a character will be a loner or an intellectual, and they will observe the action of the story from a distance, without getting too involved. We all know these types of wallflowers and, as writers, I’m guessing some of you fit this description perfectly. That’s what writers and shy kids do, they observe. While this is perfect in real life, it doesn’t work well for fiction. That’s not to say that your characters all need to be gregarious and outgoing, and you shouldn’t do away with characters who take pleasure in simply looking at the world.
. . . .
Interiority (thoughts, feelings, reactions) is your best friend here. So if a character is not taking action, give them plenty of internal reaction to keep the reader connected to and invested in their experience. Still waters should run deep. Same goes for if you’re writing an aloof or mysterious POV–it’s very easy for readers who feel distanced from their protagonist to click off.
Link to the rest at KidLit.com

“Interiority (thoughts, feelings, reactions) is your best friend here”
Agree. You need voice. You can write the most boring and/or mysterious character in the world, who never does anything except stare at walls, and, if you have voice, it could be riveting.
Love interiority. Even though my stories have action and events, there’s always more than a dash of the inner life. It’s what interests me, so of course it plays a part in the tales I tell.