Even though it has failed, inexplicably, to reach the final round for the Best Foreign Language Film award at next month’s Oscars, the Romanian film “4 Months, 3 Weeks and 2 Days” is perhaps the most warmly received new release of the year. Despite its intensely dark theme and harrowing naturalism, the film has generated a kind of ubiquitous buzz not unlike that of its cinematic polar opposite, the snarky-go-lucky “Juno.”

Both films are about a surprise pregnancy, but in “4 Months,” it occurs in a college dormitory at the close of the Ceausescu regime in Romania, when abortion was outlawed and, it’s estimated, more than 500,000 women died from illegal procedures to terminate their pregnancies. Cristian Mungiu’s film, which won the 2007 Palme d’Or at Cannes, is grim and unrelenting in its depiction of communist Romania in its latter days, situating the drama between two young roommates: the pregnant Gabita (Laura Vasiliu) and her friend Otilia (Anamaria Marinca), whose loyalty is about to be tested far more than she can imagine as she endeavors to set up Gabita’s rendezvous with a seedy, opportunistic abortionist.

The film, which opens in New York next Friday, has offered an unparalleled platform for Ms. Marinca, who thus far may only be familiar to BBC audiences. Not unlike her strong and resourceful character, the actress carries the movie on her shoulders. Captured in long, single takes in which the camera remains still and soaks in every impression, her face is an ever-shifting map of calculations, negotiations, raw nerve, paranoia, disgust, and resolve. The sensitivity to visceral emotion chafing against the imperative of keeping a cool head creates a sometimes unbearable tension, as Otilia navigates the necessities of the kind of day that could end with any of several catastrophes — and through which she endures at least two or three. Ironically, Ms. Marinca wasn’t sure which role she was going to take when Mr. Mungiu sent her the screenplay.

“I was very impressed with the beginning and the ending of the film, and the fact that he focuses on this secondary character,” Ms. Marinca said, chatting by phone from her home in London. At first, the actress’s accent seemed properly English, but as she conversed it was easy to hear the Romanian lurking amid her confident enunciation.

“It really got my attention,” she continued. “I read it during one night, and then Cristian called me to say that it was quite urgent we make a decision. I was to choose the girl to audition for.” It didn’t take long to settle on Otilia.

In his notes for the film, Mr. Mungiu recalled how he took a risk by flying Ms. Marinca from London, where she had moved after winning a prize from the British Academy of Film and Television Arts for her screen debut in the 2004 TV movie “Sex Traffic.” “I was disappointed,” the director said of his initial meeting with his leading lady. “Anamaria as a person was not my character. Next day, we read scenes together; the transformation was amazing. My character was talking through Anamaria’s mouth, as if she was possessed.”

Even though the 29-year-old actress was only a child during the time period — the late 1980s — that the film depicts, she appreciated the way the script “connected to the roots of where I am from, with the collective memory I have. Thinking about the terror in which our mothers lived, I became more and more interested.”

Ms. Marinca was born to artistic parents in northeast Romania. Her father was an actor and a college drama teacher. Her mother played violin with the Romanian Philharmonic Orchestra. Her family kept her shielded from the worst debacles of the Ceausescu era.

Even without a more explicit framework from her own experience, Ms. Marinca found it easy to sink into her role. Mr. Mungiu’s shooting style, more akin to lensing a documentary, was a significant aid. “We connected as soon as we met,” she said. “It was very easy to work with him. I knew how to tell his story. He gives you the impression that you have all the time in the world. There were no rehearsals, but in the mornings we would work until things took form, and started shooting. But it was difficult because they were so long, the scenes.”

In contrast to the totalitarian state it re-creates in psychologically suffocating detail, “4 Months” offered Ms. Marinca one of her most open and easygoing professional experiences. As the film developed, so did her character, which evolved into a more expansive role. “I was allowed to decide things,” she said. “I was creating my character, and we would do whatever we felt like doing. That was the beauty of having such a small production.”

Mr. Mungiu praised his star’s inherent gift for articulating the essence of his words. “She has a very good sense of what’s central and part of the story,” he said in a brief telephone conversation while promoting the film’s release in Tokyo. “She helped me pull out all the things that were not absolutely necessary.”

Just as significantly, Ms. Marinca possessed a transparency that made it easy for the director to simply train the camera on her for long stretches during which she had no dialogue. “There’s something special about her that I really like,” he said. “It’s not the way she delivers her lines, but the way she delivers her silences. With Anamaria’s help, we were able to transcend the limits of film. When you look at her, you can imagine what she is thinking about.”

Indeed, that quality adds immeasurably to the film’s provocative power. “4 Months” is a profoundly moral film, not in regard to the right or wrong of abortion, but in how it explores the limits of what one human being can do on behalf of another, and what some will do to exploit those who make themselves vulnerable.

Ms. Marinca, who now is concentrating on roles for the British stage, suggested that her character’s selflessness in helping the hapless and irresponsible Gabita is simply how someone would have behaved in such circumstances.

“Either you are loyal, or you’re not,” she said. “During the most difficult times in history, people stick together. Most of us are like Gabita. I would like to say that in real life I’m Otilia, but I’m not. I’m an egotist.”