Stiles gets possessed on a Thursday.
Later, he'll make a joke about it. He'll grin and say, "Never could get the hang of Thursdays," but the smile won't reach his eyes, and for a moment Scott will allow himself to viscerally hate everything that led to this, himself included.
But that's a while from now, best not to dwell on it just yet. Oh, don't worry, we'll get to that part of the story eventually, when Things have been dealt with (except for the Unspoken Things, which will not be dealt with for some time after that), but for now let's return to the point, which is: Stiles gets possessed on a Thursday.
No one notices for two full weeks.
Primarily this is due to the creature being very good at what it does. It possesses Stiles while he's alone and not expecting company for hours, giving it plenty of time to delve into Stiles' head and pull out all of his tics and habits, learn them by heart and imitate them perfectly. By the time Sheriff Stilinski pulls into the driveway with dinner sitting in the passenger seat, it could fool someone who's known Stiles his whole life.
It does just that, sitting across the table from Stiles' father and talking about his day like normal while inside his head Stiles begs and pleads for freedom, for mercy.
You see, that's the thing about being possessed. You can see and hear everything that's being done with your body, while being utterly helpless to stop it. And while sometimes possession is benevolent, resulting in nothing more traumatic than a funny story about ghosts throwing pots and getting closure with ex-girlfriends, more often it's not fun at all.
For Stiles, it's really, really not any fun at all, because while the thing possessing him is stuffing its face with curly fries, it's also mentally listing off all the ways it could torture and kill his father with only the silverware on hand. Not because it particularly hates his father. Not because it's going to kill him later and needs to come up with ideas. Just because it's bored.
It's bored, so it tells Stiles in great detail all the ways it could kill his father, if it were so inclined.
This is the kind of creature that has stolen Stiles's body.
Just to give you an idea of how those two weeks are going to feel for Stiles, let's perform a little thought experiment. Take the worst bully you ever knew - and don't front, we both know you've been bullied. Yours may not be the "jock overdosing on Haterade" cliche, yours might not have even been aware they were doing it, you might not have been aware they were doing it at the time, but you've been bullied.
So. Take the worst bully you ever knew, and imagine their voice saying all the cruelest things they ever did, all the things you were afraid they might say, taunting you and mocking you and absolutely humiliating you. Imagine that voice being set on a loop, shoved directly in your head for your listening enjoyment twenty-four/seven.
Keep that sound playing.
Now, imagine that you've been placed in a dark room, strapped into an uncomfortable chair. There's a rusty, metal contraption holding your eyes open so that you can't look away, and being projected on a screen before you is your life. Well, your life, starring that thing as you. It does a convincing job of it - everyone is fooled, and if you didn't know better you'd think this was something that happened weeks ago. Because it all looks so normal, seems so familiar.
But every once in a while, it slips. Somebody's shoulder gets dislocated at lacrosse practice because "you weren't looking where you were going." A broken beaker causes a friend's hands to get burned by acid; clumsy you. A dropped knife nearly cuts someone's toe off. Oops.
And when that happens, the loop of humiliation is broken by that thing talking to you, telling you how much worse it could have been. How easily that dislocated shoulder could have been a snapped neck, how that acid could've gotten in her eye, how that knife could've sliced his foot wide open.
How it's such a shame that it missed.
But don't worry, it won't actually happen. That thing has plans for you, and those plans mean it can't be caught in the lie just yet.
And before you can really think about just what that means, the bully's soundtrack is back on, louder and worse than before, and you're forced to watch the movie of your life play out with all these near misses happening around everyone you love.
Imagine that, for two weeks straight.
Or don't; you'll probably sleep better at night that way.