At a dinner during the Cannes Film Festival in July, Renate Reinsve found herself so nervous in the company of famous actors that she spent the evening chatting with their bodyguards instead. When a photographer who had been taking pictures of Timothée Chalamet appeared near the group, she said, her new friends waved him over.

“They were like, ‘She’s an actress, too,’” Ms. Reinsve, 34, recalled in an interview in January.

She had flown to Cannes from Oslo, where she lives, for the premiere of “The Worst Person in the World,” in which she stars as Julie, a millennial woman in the midst of a quarter-life crisis, grappling with the pressure she feels to pursue a career, find a partner and form a family. It was Ms. Reinsve’s first lead role in a film.

After some prodding, the photographer turned his lens to her. “He lifted his camera, and then he didn’t press the button,” she said. “I wasn’t worth it.”

Ms. Reinsve won the Cannes award for best actress a few days later. And in the months that followed, the film, directed by Joachim Trier, made the festival rounds, where it garnered praise for Ms. Reinsve’s performance. Louis Vuitton asked her to become a brand ambassador. Just this week she was nominated for a BAFTA in the best actress category.

At the end of January, Ms. Reinsve arrived in New York City to promote the film ahead of its American release on Feb. 4. Wearing a simple white dress and her hair in a ponytail for breakfast at Sadelle’s in Manhattan, she surveyed the tower of smoked salmon, cucumbers, tomatoes, dill and capers on the table and wondered if she would be able to eat despite her nerves. She had been up since 3 a.m., unable to sleep after she found out that she would appear on “Late Night With Seth Meyers” that evening.

“There’s been so much going on,” she said.

During the last six months, “The Worst Person in the World” has collected notable admirers, including former President Barack Obama, who included it on his list of favorite movies of 2021.

In a video interview during the Sundance Film Festival, Dakota Johnson said the movie “wrecked” her. “I was crying in a way that was weird,” she said. “I was trying to make it less than what it was meant to be. I was trying to not cry as hard as my body wanted to cry.”

The screenwriter and director Richard Curtis called the film “a complete masterpiece” in a conversation hosted by Neon, the film’s distributor, and Judd Apatow took to Twitter to say it was “stunning.”

“Renate is playing so many complex and conflicting emotions all at once and somehow we understand exactly what she is feeling in every scene,” Mr. Apatow wrote in an email. “She is able to express how hard it is to decide what you want out of relationships and out of life in a way that is alternately dramatic, romantic, heartbreaking and funny.”

For Ms. Reinsve, the whirlwind of recognition has been surreal. “I feel the same, but I feel people see me differently,” she said. “It’s a bit confusing.” She is aware of the slippery slope that intense and prolonged attention can lead to.

What makes the film good, she reiterated over two interviews, isn’t just her. It’s the script, the director and the rest of the cast (including Anders Danielsen Lie and Herbert Nordrum, who play Julie’s boyfriends). “It’s dangerous to believe that you have more knowledge, or more insight into things than other people, or you’re better than other people,” Ms. Reinsve said.

She is trying to live her life as simply as she did before all the buzz. For the most part, she does not read articles about herself.

But her father collects all the clips he can find, translates them and stores them in a file. “I’ve never seen him cry much, but this past half-year he’s crying,” she said. “He’s so proud.”

In October, Ms. Reinsve’s newfound fame and her ambivalence toward it were palpable at a party that followed the screening of “The Worst Person in the World” at the Viennale, Vienna’s international film festival. Guests tentatively approached Ms. Reinsve — at the hotel bar, in the bathroom — to compliment her performance, as well as the gold Dior suit she was wearing.

Ms. Reinsve was friendly and chatty, but as the night went on, she was drawn to the mix of salsa, pop and reggaeton playing in the ballroom. Eventually, with the help of a friend, she swapped her black heels for hotel slippers and hit the dance floor, from which she emerged an hour or so later, her blazer in one hand, skin glazed in a light sheen of sweat and hair tousled.

Existential Questions

Ms. Reinsve grew up in Solbergelva, a village in Norway that she described as more of “a road between two places.” She called her upbringing “complicated.”

“I didn’t have a good time growing up,” she said. Acting at a local theater became her solace.

At 16, Ms. Reinsve stopped going to school and left her home. She wanted to run away to Costa Rica or another warm country but could only afford a ticket to Edinburgh. There, she had enough money for one week in a hostel.

She tried to find work, but no one would hire her. Eventually, the owner of the hostel took pity. “He asked, ‘Have you ever poured a beer before?’” Ms. Reinsve said. “‘No.’ ‘But you worked in a bar?’ ‘No.’ ‘OK, but you’re over 18?’ ‘No.’ He rolled his eyes and said, ‘Fine, you’re hired.’”

Ms. Reinsve said she always felt very different from other people in her family, and that from an early age, she started asking a variation of the kinds of questions that she still wrestles with today. “Like, ‘How do people relate to each other and why?’” she said. “It kind of started happening because of my complicated relationship to some people in my life.”

As she grew older, and her relationships grew more complex, the questions evolved. “I would ask, ‘Why did I end up with this person?’” she said.

Ms. Reinsve met Mr. Trier more than a decade ago, when she was still studying acting at the Oslo National Academy of the Arts and had two lines in one of his movies, “Oslo, August 31st.”

Because Mr. Trier wanted a certain kind of light for the scene she was in, Ms. Reinsve was on set in the wee hours for more than a week. “She varied the takes and came up with 20 ideas and felt very free in front of the camera,” Mr. Trier said in a phone interview. “Most young actors at that age would have gotten lost in the toolbox.”

Over the next years, Ms. Reinsve and Mr. Trier ended up having deep conversations about love, choice and other existential matters.

“She has the star quality where you can put her in a part and she will be attractive and make the image pop,” Mr. Trier said. “But she has another dimension. She’s an incredible actor. Her drama abilities, her vulnerability in front of the camera, her ability to go deep and show complexity is what’s really interesting about her.”

Letting Go and Giving In

Right before the role of Julie came along in 2019, Ms. Reinsve had been on the brink of quitting acting altogether and pursuing a different career: carpentry.

What she didn’t know was that Mr. Trier for many years had been developing a film with her in mind. He recalled going to lunch with Isabelle Huppert, a friend, in 2017 in Oslo. Ms. Huppert was in town to see Robert Wilson’s play “Edda”; she told Mr. Trier how much she’d enjoyed the performance of an actress wearing a purple dress onstage. “That’s Renate Reinsve,” he said. “I’m writing her a film.”

Though by 2019, Ms. Reinsve had found some success in Norway’s theater scene, acting in both experimental and classical productions, she felt exhausted by the demands of the work and frustrated by the two-dimensional roles offered to her in film and television.

After buying a house in Oslo, Ms. Reinsve discovered the joys of handy work and renovation, and was ready to enroll in a carpentry program. Then came the call from Mr. Trier.

With the Cannes win and everything that has followed, carpentry has fallen to the wayside. “My house is falling apart now,” Ms. Reinsve said in a call from Norway.

Looking back, she said the decision to quit acting was somewhat freeing. “A part of growing up is just letting go of the expectations of what life should be like,” Ms. Reinsve said. “That’s something that you lose — what things could have been — and that can feel like a big heartbreak. But it’s also a relief if you go through that and just relax. When I thought that I gave up acting, it was a big relief.”

Acceptance and letting go, and all the pain and pleasure that comes with it, is at the heart of “The Worst Person in the World.” Ms. Reinsve’s Julie wrestles with universal questions: What kind of career does she want? Does she want to be a mother? How does she know when a relationship is over? What constitutes infidelity? As Julie moves through different stages of her life, she has to accept that in terms of consequences, even indecision can be a decision.

The film embraces the idea that identity is dynamic and can vacillate wildly over time. “We are always forced to try to define ourselves as one thing,” Mr. Trier said. “And none of us recognize ourselves as one thing. We are all ambivalent and chaotic.”

In portraying Julie’s decision paralysis, Ms. Reinsve wanted to dig into the messiness and show the good that can be found in a position of uncertainty. In one scene, when Julie is fighting with her boyfriend, some of her anger is driven by his need to analyze their relationship. “Everything we feel, we have to put into words,” Julie says. “Sometimes, I just want to feel things.”

Ms. Reinsve said she improvised those lines on set. “She’s unsure and she’s insecure about stuff, and that’s a good place to start,” Ms. Reinsve said. “Nowadays, you’re supposed to have a strong opinion about everything and know who you are. But then you miss out on so much of the process of becoming the you that would be a more happy being.”