Something went wrong once for Roger Greenberg. Something big.

It happened years before, and its result is as plain as every bug-eyed twitch on Roger’s face.

He’s a guy devoured by the terror of expectations. At 41, unlike his peers, he never learned to “embrace the life you never planned for.”

“Greenberg,” the latest writing-directing effort from Noah Baumbach, is another droll, sensitive essay in dysfunction.

It’s a movie that falls midway between his exquisite “The Squid and the Whale” and his woefully miscalculated “Margot at the Wedding.”

Ben Stiller leaves his silly side behind for this sometimes funny character study.

His well-to-do brother (Chris Messina) gets Roger to house-sit. Roger’s fresh out of a mental hospital. His life has been adrift, although he doesn’t like to see it that way.

“I’m doing nothing deliberately,” he insists.

But as lost, unambitious and sometimes rude as he is, his brother’s 25-year-old personal assistant sees something in him.

Florence, played with an open, downtrodden vulnerability by Greta Gerwig of “Baghead,” struggles to get by, frets over the aimlessness of her own life but finds something touching in Greenberg, something even his long-suffering college pal (Rhys Ifans) and ex-girlfriend (Jennifer Jason Leigh, who wrote the story this is based on) can’t see.

Greenberg makes passes at Florence, seduces her even.

Yet in an instant, he turns on her, lashing out with little provocation. His misanthropy is grating, his phobias (strangers, crowds) many.

It’s no surprise he’s a compulsive writer of complaint letters – to mayors, airlines, newspapers. Everybody else has to hear his complaints in person.

Stiller walks a fascinating tightrope with this guy, underpinning much of what happens with a midlife crisis – a generational angst that allows Baumbach to pass judgment on “kids today” even as his anti-hero alter ego is no one who should be passing judgment on anyone.

“I hope I die before I meet one of you in a job interview.”

Baumbach overreaches, making this character a selfish, off-putting cultural (LA) and generational scold.

But Stiller, in his most “real” performance in ages, finds the function in this catalog of dysfunctions, the humanity in this humanity-hating crank.