Thirty-six-year-old Chilean filmmaker Sebastián Silva, the writer-director and star of the comedy Nasty Baby, is himself an enfant terrible. In eight years he's made six eclectic films, his own wicked twists disguised in what looked like the usual arthouse tropes. The Maid, his American breakthrough, looked like a benign Latin American domestic drama about a servant losing her job, the kind of movie dutiful cineastes swallow like vitamins. But sentimentality is skewered dead when that housekeeper (Catalina Saavedra) tortures her replacements. Silva's subversive mimicry is like a boast that he could make a crowd-pleaser in a snap — he'd just hate himself for obeying the rules.

In Nasty Baby, Silva and Kristen Wiig take aim at a fat target: gay modern-family dramedies, those well-meaning, self-congratulatory films that haven't actually felt modern in a decade. These parlor flicks about good people reconfiguring the nuclear family share the same tics: saintly protagonists, meanie bigots and living rooms poached from Crate and Barrel. Watching them, you might wonder if the directors think that they can woo sheltered, conservative moms with aspirational decor. (Hey, that formula worked for Bravo.) Of course, sheltered conservative moms rarely see these movies. Like a Michael Moore doc, these sermonize to believers, only without the fire or wit that would actually make them fun.

Nasty Baby feigns that it fits that banal mold. Wiig plays Polly, a Brooklyn spinster desperate to bear her gay best friend's baby. (“Spinster” is a loaded word, but Silva and Wiig make Polly so asexual that she never even glances at a passing hunk — heterosexual mating is as unthinkably impossible for her as suddenly deciding to build her own rocket to the moon.)

The pal, Freddy (Silva, in his first major role), is a performance artist in a long-term relationship with Mo (Tunde Adebimpe), an introverted furniture designer. Yes, their apartment is fabulous. Freddy is thrilled to donate sperm. He's as broody as a hen and mature enough, mostly, to grasp the consequences. But Silva makes him a touch fatuous. Diapers don't occur to Freddy. He's half in it just to make an art project about passing on his DNA.

After six months of awkward dinner parties where Freddy decamps to a bathroom while Polly impatiently ovulates, the would-be dad is stricken to learn his sperm count is too low. He and Polly turn their gaze to third wheel Mo, who's much less enthusiastic about filling up a baster.

All this setup suggests a virtuous dramedy, with Mo weighing the risks of a yea or nay. Here's what would concern us in most movies: If Mo and Freddy split up, Mo would still be bound to Polly for life. But does he want to die alone, like the busybody gay widower (Mark Margolis, great) on their block, who underscores his purpose as an example by chirping, “I'm the ghost of Christmas past”?

These questions are important but turgid — the stuff of ethics classes, not people. Silva ignores them and centers his film on his characters' flaws: Polly's ruthless pressure, Freddy's condescending eye-roll when Mo's erection wilts from anxiety. The best friends team up and treat Mo like a stud bull. “I want it!” Polly whines, thrusting her ass in Mo's face. When Mo's family — the would-be grandparents and uncles — raise their concerns, Polly is as aghast as if they'd donated to Rick Santorum. Meanwhile, Freddy continues shooting his latest gallery submission, a snippet in which he sobs like an infant. It, too, is called Nasty Baby, both the film and the film within the film italicizing his immaturity.

The performances are marvelous. Silva's Freddy is delightfully maddening; Adebimpe's Mo moves like a barge floating above the storm.

As for Wiig, the off-kilter comedian has been leaning toward drama for years, which is as big a waste of her talents as asking an opera singer to rap. In serious roles, she keeps her lips tense and her voice clipped, as if she mistakes shrinking herself for sincerity. But who wants her to take up less space? We want her big and loud, and here she finally splits the difference between SNL caricature and square, creating a woman who's desperate, obnoxious, loving and complex — i.e., that rarest of movie creatures, a real girl.

Watching the three ping-pong off one another's moods is fun. Yet I waited impatiently for Silva to build the tension into something substantial, preemptively annoyed at the moral I was expecting. My mistake. Life isn't made of important decisions settled calmly over cappuccinos. We evolve haphazardly, tiny fluctuations in temperament triggering abrupt changes — one too many glasses of whiskey, one unforgivable insult, one forgotten condom. Like Darwin's dodos, maybe we'll survive our mistakes, or maybe we won't.

Silva subverts the setup of his own script to get to these shaggier, messier truths about human choices, and if the film itself is as sloppy as life, so be it. At the end of the film, we've exchanged a life for a life. The audience spots the symbolism. The characters don't. Nasty Baby isn't satisfying. But on Silva's terms, it makes sense.

NASTY BABY | Written and directed by Sebastián Silva | The Orchard | Sundance Sunset

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