Greta Gerwig in Noah Baumbach's "Frances Ha" Greta Gerwig, with Noah Baumbach, who directed her in "Frances Ha," a black-and-white portrait of a free-spirited New Yorker in her 20s. An address on Vanderbilt Avenue in Brooklyn. An F train that never comes. Unfinished on-spec scripts for "Saturday Night Live." The artistically inclined friend of yours who, incongruously, is dating a man who works at Goldman Sachs. That exorbitant $3 fee at the Bank of America A.T.M. - and the long unpalatable walk to the next deli, to find a cheaper option. Such are the totems of young life in New York City, the visual and emotional shorthand that anyone who has lived and struggled here in their underfinanced 20s will immediately recognize. This period of time - the years of going to adult dinner parties without anything adult to relate; the friendships that end over trivial matters of geography and relationship status; the afternoons of waking up at 2 p.m., and starting to drink at 3 - is the subject, loosely, of a heartfelt new film from Noah Baumbach, "Frances Ha." On a recent Thursday, Mr. Baumbach and Greta Gerwig, Mr. Baumbach's writing partner and lead actress on the film, were having a late breakfast in the West Village. Both had come from exercising - Ms. Gerwig, 29, freshly showered after a trip to the gym, her complexion as bright and uncomplicated as a lighthouse; Mr. Baumbach, 43, in a post-yoga plaid shirt and pinstripe suit, face framed by two neat swoops of black hair. For a moment, the scene could have been out of their movie: a workday on which no one is really working; a room full of beautiful people saying inarticulate yet strangely specific things to one another, like Ms. Gerwig's drink order, tea sans sugar or milk. "Black but green," she told a momentarily bewildered waiter. The illusion that the two were aimless characters from their own film did not last long. "Frances Ha" is about a 27-year-old chrysalis of a human, played by Ms. Gerwig. Frances is an aspiring modern dancer conducting a platonic love affair with her best friend, Sophie (Mickey Sumner), and the New York City - shot in seductive, "Manhattan"-reminiscent, digital black and white - they both live in. It is a deeply romantic film with no real romance at its center, a love letter to a city that is depicted, at times, as anything but lovable. "There's no heterosexual relationship in it," Ms. Gerwig said. There's no kissing. Instead, the movie chronicles Frances" flirtation with adulthood and her subsequent fall from perpetual adolescent grace. "I'm so embarrassed," Frances says. I'm not a real person yet. By contrast, Mr. Baumbach has made a living writing and directing critically adored films about people like Frances since his 1995 debut, "Kicking and Screaming," made when he was just 25. And Ms. Gerwig is one of the most fascinating actresses of her generation, known for her seemingly unstudied performances and nervy physicality. Neither of them lack for accomplishments, or a sense of purpose. For Mr. Baumbach, "Frances Ha" is a return to a moment he's never really stopped writing about: "That period in your 20s where you're necessarily having to separate yourself from a kind of romantic idea of yourself," as he put it. But his heroes, or antiheroes, have always been borderline unpleasant: Ben Stiller's painfully maladroit Greenberg, in the 2010 film of the same name; the self-aggrandizing patriarch played by Jeff Daniels in Mr. Baumbach's semiautobiographical 2005 film "The Squid and the Whale"; Nicole Kidman's scarily neurotic Margot, of 2007"s "Margot at the Wedding." The Frances of "Frances Ha," by contrast, is a danger only to herself: she needs to discover not how to stop hurting other people but how to stop other people from hurting her. She's the most sympathetic character Mr. Baumbach has put on screen, and among the most alive, too. That fact is in part a credit to Ms. Gerwig's performance, the most detailed iteration of a character she too has played before: someone young and exposed and not quite fully formed. Mr. Baumbach and Ms. Gerwig met on the set of "Greenberg," in which the actress played Mr. Stiller's guilelessly vulnerable love interest, Florence, in a performance so vivid that in certain precincts of Hollywood, Ms. Gerwig said, she is still mistaken for the character. "People thought that was me, especially when people in L.A. saw it," she said. And then I would have meetings with them, and they would speak to me in a high, soft voice. They would say, "Oh, it's so nice to meet you," like I was going to just burst into a puddle." After "Greenberg," Mr. Baumbach sent Ms. Gerwig an e-mail. There was something "authorial" about her acting in "Greenberg," Mr. Baumbach said, that made him want to work with her again. He was searching for his next project, which he knew would be set in his native New York, and which he was planning to make in a more guerrilla fashion than his last few films. "Something that was stripped down and fleet-footed," he said - no real Hollywood stars, a smaller budget, shot on location (and, at times, in Ms. Gerwig's actual apartment). He wrote Ms. Gerwig, asking for ideas. The two began a collaboration and - though neither will really talk about it - a romance.