Natalie Wood gazed off-camera, her moonlit eyes glittering with sexual speculation, and wondered aloud what James Dean was like. "Well, you have to get to know him," Sal Mineo replied. His face, turned in the same direction as Natalie’s, glowed with similar lust but even greater candor. His words, inadequate to the great subject that had been proposed, emerged in dreamy, indefinite phrases, until at last he hit on the right one: "He’s sincere."

"That’s the main thing," Natalie said.

So we learn the lesson of Rebel Without a Cause: the main thing in life is not to be honest, truthful, plainspoken or straightforward but sincere. Why this should be so is not self-evident; but, if true, what can the movies tell us about this goal? Set aside all the evidence that the memoir writers have been piling up; pass by the equally mountainous body of handmade documentaries about My Personal Journey, and the pooling televisual tears of Glenn Beck. Let’s try to discover something about sincerity by looking just at movies–or, to begin, at movie stars.

I think of poor, trembling, heart-in-throat Judy Garland, who was as sincere as a whistling teakettle. Jimmy Stewart from his great, slender height could give off sincerity the way a tree drops leaves, fumbling and stammering it from himself. And what about Marilyn, that uncanny transmitter stuck in the "on" position, whose signal of vulnerable, innocent warmth somehow cut through the static of hair dye, falsies and elocution? It’s sincerity, as much as sex appeal, that has linked her in a kitschy afterlife with James Dean. Sexy actors and actresses lie thick in Forest Lawn; but sincere ones are exceptional in a business that values controlled persuasiveness, not the seemingly involuntary broadcasting of weaknesses and wounds. That’s the first thing to be learned. Go beyond the few stars mentioned, and you soon find yourself straying from Hollywood into the emotive world of John Cassavetes.

Here we come to the second lesson, which is to distinguish between sincerity, on the one hand, and ideals such as individuality and authenticity, on the other. Many directors have made personal films without any show of laying bare the heart. (Think of Erich von Stroheim, who had "phony" for a middle name. Think of the wily Hitchcock.) And by the 1950s, thanks to the prestige of Italian neorealism, directors who wanted to demonstrate their seriousness could do so by exhibiting a rough, seemingly spontaneous style. It was Cassavetes, more than anyone else, who went beyond these precedents, to take sincerity as his method and subject matter.

This great development in the history of sincerity could not have been achieved without a fight–which was fortunate, since Cassavetes was always spoiling for one. To shape the filmmaking around the actors, rather than make the actors conform to the filmmaking, required a revolt that was as much commercial as aesthetic. A successful revolt: I don’t think we’d be misreading film history too grossly if we were to trace back to Cassavetes a vast celluloid river that’s been flowing in America for the past forty years, swollen with cheaply made studies (they don’t always have enough plot to be dramas) about ordinary-seeming people whose main business is to express themselves without affectation. It would not necessarily be a disparagement of these films, some of which were recently tagged as a not-quite-movement called mumblecore, if I were to point out that by now they require no revolt. A third lesson: sincerity may never become big at the box office, but it long ago claimed its own market niche.

Which leads me to Noah Baumbach’s Greenberg. It’s more than watchable, thanks in part to the casually luscious cinematography by Harris Savides. You might enjoy passing a couple of hours with it; and if you happen to be particularly curious about movies, you might even go out of your way to see it, knowing that Baumbach is, among his other credits, the writer-director of The Squid and the Whale (that exceptionally literate and astringent comedy of domestic woes) and the co-writer, with Wes Anderson, of Fantastic Mr. Fox. He’s worth following; but that’s not why I bring Greenberg to your attention. I’m interested in this beguiling, outwardly modest film for what it suggests about the difference between the personal and the sincere, as summed up in a flirtation (or is it a conflict?) between two generations.

In the title role, playing a character of approximately Baumbach’s age, is Ben Stiller, the sharp planes of his face looking more chiseled than usual within a frame of shaggy hair. Some generational exemplar, this gaunt and messy Roger Greenberg. All you know about him at first is his fragility: his lack of a job, his recent stay in a psychiatric hospital, his diverse supply of prescription drugs. As the story begins, he has returned from New York to his native Los Angeles, where he will pass the time house-sitting for his well-to-do vacationing brother, evidently having nowhere else to go right now and no better use for his trade as a carpenter than to build a doghouse for his brother’s German shepherd. More information will be forthcoming (Baumbach is good at playing out a backstory); but this is enough for now, given Roger’s unblinking stare and his way of moving as if swaddled in cotton. Most people, by the time they hit 40, have tasted disappointment. Roger, you can see, is woozy from gorging on the stuff.

Cast as his opposite, and coming from the generation still in its 20s, is a veteran of the mumblecore scene (the veteran having been born in 1984, the scene arguably having peaked in 2007), Greta Gerwig. She plays Florence Marr, salaried errand girl for the entire household of Roger’s brother: a young woman so unassuming that she feels she’s been granted a favor when, on the road, she gets to change lanes; so meek that she pretends to be not merely resigned but pleased when Roger’s brother, on his way out of town, says he doesn’t have a paycheck for her right then. "It’s better in a way," she replies, "because then I don’t spend it all at once," as if she had anything to spend at all.

So Baumbach sets up the meet-cute. Blond and baby-cheeked Florence, with her floppy hair, floppy clothes but too readily upturned lips, will have to pass by the brother’s house to pick up her belated paycheck, and so will run into Roger. For two or three minutes, they will stand on either side of a kitchen island and use their eyes to perform an elaborate dance of mutual avoidance.

What exactly are these people trying to evade? To say sex would be improbable, given that this is a modern romantic comedy. Bodies will collide (though Greenberg does break with convention by accelerating from zero to wreckage-strewn disaster in less than ten seconds). A more likely answer might be that Florence and Roger hope to escape embarrassment (and will fail–see above). But there is something else to be avoided within the film’s milieu of pricey homes and show-business careers, something that repels both characters so strongly that it helps push them together. Roger and Florence are determined to keep their distance from ambition.

This fecklessness seems wholesome in the well-scrubbed Florence, who (unlike Roger) has enough energy and competence for both work and play. When not picking up the dry cleaning for the brother and walking his dog, she moonlights as a singer–though not as if she wanted to command anyone’s attention. Performing in the kind of bars where you’re expected to bring your own audience, Florence dedicates herself to a repertory of the most exquisitely refined obscurity, singing as if to herself while her hands wander in search of a resting place. All right, it’s a style. But this is also her sincerity: her refusal not just of careerism but of any effort to create a public voice, even when one seems to be called for. Her art, a direct extension of her body, goes no further than immediate earshot. If she entertains a troubled and probably inappropriate attraction to Roger, it’s partly from her notion that he, too, doesn’t want to impress people in general or chase after success. He’s alluringly undemonstrative.

And yet, as the audience knows but she doesn’t, Roger’s refusal of ambition is blatantly insincere. Although he volunteers to his old friends that he is "trying to do nothing for a while," as if this were a spiritual discipline, he has been furiously attempting to reform society by means of letters of complaint, which he addresses to a roster of corporations and political bodies that have not met his expectations. And what high expectations they are! His other main occupation, which he seems to have pursued for the past fifteen years, is to maintain himself in the purity of the long-ago moment when the world failed to give him exactly the career he wanted. Let others of his generation settle down to unworthy endeavors. Roger will remain a proud case of arrested development.

This fecklessness seems pathological–but in its self-contradiction, self-involvement, duplicity and emotional violence, it opens possibilities that are unknown to Florence’s sincerity. Unlike Florence, Roger is trying to find his public voice. Over the course of the film, starting from a condition in which he can scarcely stammer a few shaky words, he will succeed, as his raging need to tell the world how to conduct itself becomes steadily more open, articulate, sarcastic and locally targeted. In a sense, Greenberg is driven forward not by narrative complications but by a mounting urge toward vituperation, which at last bursts out against an entire house party full of people around Florence’s age.

They’re all so sure of themselves. She alone of her generation is exempted from the tirade–first, because Roger now knows she’s vulnerable, and, second, because she’s resting at home and so is out of the primary danger zone, which is whatever happens to be in front of his eyes. When the fit passes, and his impulses are as close to being in control as they get, Roger will think again about his feelings and address Florence kindly, as an individual. What’s telling, though, is the way he does it. To speak to the woman who put the "mumble" in mumblecore, the one who seems to want to be heard only by those who are close at hand, he resorts to mechanical reproduction, leaving a voicemail message.

So we learn a lesson or two from Greenberg.

First, we may conclude that the therapeutic theme of this sincere movie is not to be trusted. Yes, it appears as if Florence’s role is to heal Roger. (And I mean "appears": to see them side by side is to see a tall glass of fresh milk next to a twist of beef jerky.) It also appears, on a more allegorical level, that Gerwig, as the emissary of a younger art-house generation, has been recruited to rejuvenate Baumbach, now that he is hitting middle age and leaving a clear auteurist signature (that is to say, starting to repeat himself). But it’s youth in Greenberg that flops backward for age, from the first grope to the final acquiescence. As sick and pathetic as Roger is made out to be, he’s the one with the more powerful language, the louder voice, the pushier attitude, the wider frame of reference; and he’s not alone among his generation. Jennifer Jason Leigh is here, too, in a small role (in addition to having thought up the story with her husband, Baumbach), as if to remind us of everything that the lead actress is not. Gerwig is a warm and amiable screen presence in Greenberg; but when you compare her with Leigh, the sincerity of a younger generation is overmatched by the personal power of an older one.

But then, as the final lesson, we should learn not to trust the persona that Gerwig puts on for Greenberg. This is a woman who has pushed herself to play roles in fourteen films to date, plus write, direct and produce, all before seeing the dark side of 27. Some generational exemplar; some harmless floater. It’s actually the older, unashamedly prickly filmmakers of Greenberg, now secure in their success, who have put forward the image of a sincere Gerwig, for the sake of making a consoling story about bighearted losers.

That’s pretty dishonest–but in its way, it’s also kind of sweet.