I haven't conducted a study or anything, but it seems to me that when people dealing with mental illness lose control in real life, it tends to be less like an extravagant production and more like a quiet one-person show. The one person I know who really went for it called me in the middle of an episode to talk to me about math and about how the government was out to get her. Her family helped her check into a hospital not long after that. Happily, she's fine now.
But in movies the final result of just about everybody's descent into that kind of confusion always turns into stabbing ballerinas or buildings on fire. Showing is easier than telling. Which makes a story about a man quietly losing his grip, the way a real person might, a lot more difficult to convey.
And that is why you cast Michael Shannon.
Shannon is a talented character actor who's become more and more known for slightly (or more than slightly) off-kilter characterizations. He can make his eyes move in two different directions, which helps a lot, but mostly he's just got the twitchy, creepy, something-bad-is-coming moves down cold. If you saw the freaked-out indie movie Bug, he was the mentally ill man who convinces Ashley Judd that they're both infested with invisible insects. They lock themselves in a motel room, he performs a little home dentistry on himself and then they torch the place. Fun.
Here Shannon plays a working-class family man with a contented life and no visible tics. He isn't wild-eyed. He doesn't shout or rant. His wife, Jessica Chastain, is happy and together they're raising a deaf daughter who's about to undergo cochlear implant surgery. But Shannon's having bad dreams about impending storms and shadowy strangers attacking him and his family. The dreams become more vivid and more anxiety-producing and daytime visions of swarming birds and the sound of thunder that only he can hear makes him worry that he's inherited the same mental illness that caused his mother to be institutionalized in her thirties.
This Average Joe is torn between what he knows to be the facts -- that he's experiencing symptoms of mental illness -- and what his feelings tell him about real, approaching, physical danger. So he consults with doctors and he sets about the obsessive task of building a storm shelter to protect his family from the apocalypse. He wonders if and when he'll go full Black Swan and at the same time tries to build a physical structure to contain the external disaster that his delusions tell him is truly coming.
It's a domestic drama and a horror movie that want it both ways, too. It loads small actions full of meaning and dread for one man -- you know, for example, that borrowing heavy equipment from the job site to build that storm shelter is going to result in some negative consequences -- and it reaches for universal feelings of shared modern panic and fear. And the ambiguous final moments collapse the opposing sides, so that it's about one man melting down and also about the whole real world outside the movie theater going crazy at the same time. And that's why, unlike with a louder, shriekier movie like Bug, you can't just leave after the credits and shake this one off.