Comparables: Signs, Pitch Black, the Tim Robbins section of War of the Worlds, Alien
Cat and mouse stories have many permutations, and they all work. A Quiet Place is one of the more literal, primal takes on this yarn: the human characters are mice, scurrying away from larger apex predators. Despite the simplicity of this mechanism, the movie loses steam in the later portion. You'd think that instant access to mortal danger would help the storytelling, but these sonar monsters are on screen too much. They lose their menace and become video game minibosses, with the perfunctory weakpoint, to be solved with a particular combination of inventory items.
Also, the way the characters get flung into danger gets a bit funny after awhile. Murphy's Law -- that which can go wrong, will go wrong -- is really a law of comedy. I loved the nail on the staircase, particularly because it was set up earlier, and paid off during the already suspenseful delivery scenario, but the trapdoor fall into the grain silo and the flooding basement were two too many for me. There's only so many times I can watch characters hold their fingers to their lips, bug-eyed, or watch Emily Blunt tremble with fear as a monster sniffs around her.
It's not surprising the movie peters out a bit: stories this high concept tend to. But here's a story that also happens to have all its emotional punch up front, with the death of the third child. That opening scene is why the movie did so well, I think. I stopped noticing all the foley work once that kid died. After that, the jeopardy to the family felt credible, and there was some emotional energy to work with.
This is the kind of story where everybody gets one arc, and I think the writers succeeded in only two of the four. The parental grief played great for me -- losing a child in a survival situation that extreme is resonant. They took the right amount of time to show the parents mourning. The daughter's guilt over handing her baby brother the fatal toy didn't quite land me, and the son's acceptance of his duty to perform masculinity on the frontier was only so-so.
There's too much movie after the childbirth, and the only piece of drama is the father proving to his daughter that he loves her by dying for her. This doesn't take full weight because there is no surprise there: we know he loves her. One skirmish about whether or not she can enter the basement doesn't convince us otherwise. So when he lets out his final yawp, I had a hard time sharing in her emotion. (I actually thought: "Smart, Krasinski, give yourself less shooting days so you can focus more fully on directing.")
Like its comparables, Signs and War of the Worlds, the solution to the alien menace is fairly banal. The interesting thing, though, is that there has to be a solution, because this movie takes place on the homefront. If there isn't a way to beat the monsters, then our protagonists will have to live like mice until they die a miserable death, probably not long after the credits finish rolling. In a movie like Alien or Pitch Black, it doesn't matter if there's any trick to defeating the monster: you win by getting the hell out of there (and then nuking it from orbit). Which is why Alien only picks up momentum as it goes: that goal of get out, survive is so crystal clear, and always urgent. And because the monster is indomitable (not to mention better-designed), the threat remains high. Alien also had more surprises than A Quiet Place. The characters (and the audience) are learning new things all the time. Things like: oh wow, that guy had an alien gestating in his thorax, or: oh wow, that guy was actually an android, or: oh shit, this company thinks they can capture this alien. The family in A Quiet Place knows they need to shut the fuck up, and that doesn't change.
If this were my script, here's how I would have revised it. Treat the childbirth as the finale. To make that work, I'd age up the surving son, Marcus, to 13 or so, and kill off the father earlier. This pays off the "who are we if we can't protect them" moment between husband and wife, and thrusts Marcus into the protector role in a more dramatic way. But I wouldn't play the father's sacrifice as heroic -- there would be a suggestion that he's taking the easy way out, succumbing to the desperation of their situation. Seeing this, Marcus is fully inspired to embrace his survival instinct. To give his character even more heft, I'd have him, rather than the father, blame Regan (the deaf girl) for their younger brother's death, and spend some time proving that. So the final act of the film would look like this: mom is out of commission and vulnerable as she's delivering, their protector is dead and gone, and the two siblings have to save each other so they can save mom. They kill off the monsters, and in the last scene they enter the delivery room to meet their new baby brother, healthy, wailing, and not in any danger because of it.