Everyone knows about development hell, but there’s also a post-production hell, which is where Kenneth Lonergan’s follow-up to his well-received “You Can Count On Me” has been residing for more than a half-decade. Disagreement between the writer-director and the distributor over the final cut apparently kept “Margaret” on the shelf since 2005. (Both producer Sydney Pollack and executive producer Anthony Minghella are deceased.) Now it’s finally been released in a 149-minute cut (being just short of 2½ hours was apparently a requisite), and though a bit choppy, it emerges as a thoughtful, compelling piece about guilt, redemption—and the difficulty of coming to terms with the moral complexity in human affairs. Some may call it epic in the sense of length alone, but it’s more accurate to call it universal in terms of its themes and message.

Despite the title—which reflects a melancholy poem by Gerard Manley Hopkins—the chief character is Lisa Cohen (Anna Paquin), an intense, self-centered, precocious New York teen who’s outspoken in her judgments and living with her actress-mother Joan (J. Smith-Cameron) and younger brother in an Upper West Side apartment. Her divorced father (played by Lonergan himself) lives out west and has remarried, but they talk by phone fairly regularly.

In fact it’s while preparing for a visit to him, when they plan to vacation at a ranch, that the incident that drives the plot occurs. While out shopping for a cowboy hat for the trip, Lisa spots one on a bus driver named Maretti (Mark Ruffalo), and in trying to catch his attention she distracts him from his driving. He runs a red light and hits Monica Patterson (Allison Janney), who dies in Lisa’s arms. But wracked with guilt herself, Lisa tells the cops that the light was green, sparing the driver trouble with the law.

Though she tries to put it behind her, the experience gnaws at Lisa as she argues angrily with other students—particularly about Islamic violence—in her History class, as well as in the English class where nebbishy John (Matthew Broderick presides. She’s also having trouble in the Math class taught by straight-arrow Mr. Aaron (Matt Damon).

“Margaret” is replete with ancillary plot-lines. Lisa cavalierly dismisses the attentions of Darren (John Gallagher, Jr.), a geeky classmate who’s clearly infatuated with her, but chooses to lose her virginity to Paul (Kieran Culkin), a cooler, more ostensibly mature one—who even smokes! And she comes on to Mr. Aaron, while her long-distance relationship with her father begins to deteriorate. Meanwhile Joan launches a relationship with Ramon (Jean Reno), a Colombian businessman who admires her onstage and hesitantly approaches her off it. He also introduces her to opera at the Met.

But the major thrust of the script is Lisa’s decision to try to reopen the accident case. She approaches Maretti, who’s understandably anxious about her effort (as is his wife, played by Rosemarie DeWitt), and then the police, whom she manages to antagonize. She also asks Monica’s best friend Emily (Jeannie Berlin), an intense woman, for help, and the result is a civil process in which Monica’s distant cousin becomes the plaintiff. Lisa’s purpose is justice—she wants Maretti to admit his mistake and be taken off the road. But she finds that others involved in the legal process have different motives, and that law and justice aren’t necessarily bedfellows.

To be honest, “Margaret” doesn’t tie all these threads together seamlessly. But Lonergan doesn’t intend to. The messiness of the film is a reflection of the messiness of life, especially as seen by a high-minded but confused young person desperately seeking answers who finds that the adults from whom she seeks guidance are ambivalent, hesitant or unhelpful. And her confusion is exacerbated by unexpected emotional eruptions she can’t even begin to fathom. There’s no easy or comfortable resolution to the film, though there is a very personal reconciliation between mother and daughter to the strains of Offenbach’s Bacarolle that indicates the possibility of maturing in the face of the world’s imperfections—and the reality of loss, death and disappointment.

Most of the supporting cast, including Lonergan himself, respond to his unhurried directorial pace with naturalistic performances that are quietly truthful and unshowy. A few—notably Berlin, Janney and Smith-Cameron—have big moments that they savor. And Paquin centers the film with a fearless turn that shows she’s unafraid to play grating and unsympathetic; her bedroom scene with Culkin is remarkable, but it’s only one of many strong moments. Technically the film captures a world that looks authentically lived-in, even in posh environs like Lincoln Center, with Rysznard Lenczewski’s cinematography emphasizing reality over any prettifying impulse.

“Margaret” will be too languid and discursive to appeal to viewers impatient to get on with things and wanting a clear-cut conclusion. But in its concern with significant ethical issues and its unforced, realistic style it resembles an American cousin to the films of Jean-Pierre and Luc Dardenne—though it deals with people on a higher socio-economic level, of course.