In 1978, a young Meryl Streep was on the verge of becoming the greatest actress of her generation. She was also about to lose the love of her life.

“She doesn’t talk about it much,” says Michael Schulman, who explores this time in his new biography “Her Again.” “But that year was so wildly eventful and dramatic in her life. It was instrumental in shaping who she was as a person and an actor.”

Streep was 29 years old, a gosling in the New York theater world. She was living in a loft on Franklin Street with her boyfriend, actor John Cazale. He was 14 years her senior and a legend among his peers.

“I learned more about acting from John than anybody,” Al Pacino has said. “All I wanted to do was work with John for the rest of my life. He was my acting partner.”

Streep and Cazale met in 1976, when they were cast opposite each other in “Measure for Measure” in Central Park. By then, Cazale was not quite a star — he lacked that ephemeral quality — but he was regarded, in the industry, as a rare talent, in demand among the great directors of the era.

He was Fredo in “The Godfather” and “The Godfather Part II,” and had lead roles in “The Conversation” and “Dog Day Afternoon.” Of the five movies he starred in, all would be nominated for Best Picture, and three would win.

“One of the things I loved about the casting of John Cazale,” said “Dog Day” director Sidney Lumet, “was that he had a tremendous sadness about him. I don’t know where it came from; I don’t believe in invading the privacy of the actors I work with or getting into their heads. But my God, it’s there — every shot of him.”

“Time moved differently for John Cazale,” Schulman writes. “Everything went slower. He wasn’t dim, not by a long shot. But he was meticulous, sometimes maddeningly so.”

He was known among directors as “20 Questions,” because he wanted detailed back story on all his characters. Pacino said a simple dinner with Cazale would become epic: “I mean, you’d be done — washed, finished and in bed — before he got halfway through his meal. Then the cigar would come out. He’d look at it, light it, taste it. Then finally smoke it.”

And then there were his unusual looks, so perfect for the misfits of ’70s cinema: attenuated frame, high forehead, prominent nose. Sad, black eyes.

Streep fell for him instantly. He was equally knocked out.

“Once he was in that play,” actor Marvin Starkman said, “the only thing he talked about was her.”

In looks and manner, Cazale was completely foreign to the young Streep.

“He wasn’t like anybody I’d ever met,” she said later. “It was the specificity of him, and his sort of humanity and his curiosity about people, his compassion.”

Of the two, Cazale was the famous one, but they were still starving artists. Cazale would take Streep to dinner in Little Italy, where restaurant owners, awed to have Fredo in the room, insisted they eat for free.

“They were great to look at, because they were kind of funny-looking, both of them,” said the playwright Israel Horovitz. “They were lovely in their way, but it was a really quirky couple. They were head-turners, but not because, ‘Wow, is she a beauty!’ ”

As Schulman writes, “the romance moved as fast as John moved slow,” and they were soon living happily together in Cazale’s Tribeca loft.

They were the envy of the New York theater world — she the most naturally gifted actress in generations, he the most naturally gifted actor, legendary director Joe Papp their patron — until one day in May 1977.

Cazale, who was in previews for “Agamemnon” uptown, had been feeling ill enough to miss performances. Papp was concerned enough to get Cazale an emergency appointment with his own doctor on the Upper East Side.

Within days, Streep and Cazale were sitting in the doctor’s office with Joe and Gail Papp. The diagnosis: Cazale had terminal lung cancer. It had spread throughout his body.

They sat there, Gail Papp said, feeling “like you’ve been struck dead on the spot.”

“John fell silent,” Schulman writes. “For a moment, so did Meryl. But she was never one to give up, and certainly not one to succumb to despair . . . She looked up and said, ‘So, where should we go to dinner?’ ”

Cazale dropped out of his play immediately. Streep was starring in the musical “Happy End,” and her castmates saw no sign of her anxiety or grief. Cazale would show up at the theater now and then, still smoking his cigars. Streep didn’t nag or criticize — she simply made her own dressing room off-limits to smokers. Her grace outstripped her age.

“She had a kind of tough love about it,” actor Christopher Lloyd said. “She didn’t let him malinger.”

Streep and Cazale tried to keep the severity of his condition between them. Even Cazale’s brother, Stephen, didn’t realize how bad it was until one day, after the three of them had lunch in Chinatown, Cazale stopped on the sidewalk and spat up blood.

Al Pacino took Cazale to radiation treatments, sitting in the waiting room, hoping it wasn’t as bad as it seemed. Cazale himself insisted he’d get better, and when he fought to go back to work, Streep took a part she loathed just so she could be with him.

She was just “the girl” in the movie — “essentially a man’s view of a woman,” Streep said. “She’s extremely passive, she’s very quiet, she’s someone who’s constantly vulnerable.”

In short, she was everything Streep was not. But the film was “The Deer Hunter,” and Cazale had the chance to star opposite Robert De Niro in one of the few films to then grapple with the Vietnam War.

The filmmakers fought to cast Cazale, even as the production company, EMI, insisted he be fired: The insurance costs would be outrageous, and no one wanted to back a movie with a terminally ill star.

“I was told that unless I got rid of John, [they] would shut down the picture,” director Michael Cimino said later. “It was awful. I spent hours on the phone, yelling and screaming and fighting.”

The story Streep would later tell: De Niro covered Cazale’s insurance costs, which the actor has never confirmed or denied. “He was sicker than we thought,” De Niro later said, “but I wanted him to be in it.”

They got through the shoot. All Streep wanted to do was quit working and be with Cazale, but they were struggling under medical bills. Reluctantly, she took a lead role in the nine-hour TV miniseries “Holocaust” — the “Roots” of World War II — solely for the money.

“Holocaust” was filmed in Austria, and Cazale was too weak to go. Streep never complained — Schulman describes her as embodying “cheery professionalism” — but quietly she agonized.

“The material was unrelentingly grim,” she later said. They shot on location at an actual concentration camp, which she found profoundly disturbing.

Shoot days were added to the schedule. She’d spent 2½ months in Austria, longer than she’d been told, separated from her dying boyfriend, each day another lost forever.

“I was going crazy,” she later said. “John was sick, and I wanted to be with him.”

‘When I saw that girl there with him like that I thought, “There’s nothing like that.” As great as she is in all her work, that’s what I think of when I think of her.’ - Al Pacino

“She was very anxious to do her very last scene and then zip back out,” director Marvin Chomsky said. “I mean, I don’t even think we had a moment to say goodbye.”

When Streep got back to New York, Cazale was worse than she’d ever seen him.

For five months, the couple disappeared, and for Streep, this was it: No more work, only Cazale. His cancer had spread to the bones, and he was increasingly weak. She went with him to every doctor’s appointment, every radiation treatment and never betrayed a lack of hope.

“She was always a strong-willed, persistent, hopeful person, and I think she just applied all her spirit and strength to taking care of him,” Schulman says. “She wasn’t one to create drama around it or draw attention to herself. She just bore down and did what needed to be done.”

Streep later said that the time they had together, retreating into their cocoon, gave her a weird sort of protection. “I was so close,” she said, “that I didn’t notice the deterioration.”

She confided in very few people, and wrote to her old drama teacher at Yale, Bobby Lewis, of her true emotional state.

“My beau is terribly ill and sometimes, as now, in the hospital,” Streep wrote. “He has very wonderful care and I try not to stand around wringing my hands, but I am worried all the time and pretending to be cheery all the time, which is more exhausting mentally physically emotionally than any work I’ve ever done.”

In early March 1978, Cazale entered Memorial Sloan Kettering. Streep never left his side. On March 12, 1978, at 3 a.m., Cazale’s doctor told Streep, “He’s gone.”

“Meryl wasn’t ready to hear it, much less believe it,” Schulman writes. “What happened next, by some accounts, was the culmination of all the tenacious hope Meryl had kept alive for the past 10 months. She pounded on his chest, sobbing, and for a brief, alarming moment, John opened his eyes. ‘It’s all right, Meryl,’ he said weakly. ‘It’s all right.’ ” Then he closed his eyes and died. Streep’s first call was to Cazale’s brother, Stephen. She sobbed throughout.

“I tried,” she told him.

That year, Streep had success upon success: She won an Emmy for “Holocaust,” an Oscar nomination for “The Deer Hunter,” was cast in a career-making role in 1979’s “Kramer vs. Kramer” — which won her the Academy Award — and was cast as Kate in Shakespeare in the Park’s “The Taming of the Shrew.” She had become a star.

But Cazale’s death, and her own suffering, had transformed her, as a person and an actress.

She had a new, different interpretation of Kate: She was not an independent woman to be broken by a man but one who learns the deep satisfaction of giving herself over to love.

“What I’m saying is, ‘I’ll do anything for this man,’ ” Streep told a reporter at the time.

“Look, would there be any hang-up if this were a mother talking about her son? Service is the only thing that’s important about love. Everybody is worried about ‘losing yourself’ — all this narcissism. Duty. We can’t stand that idea now either . . . But duty might be a suit of armor you put on to fight for your love.”

For all her later accomplishments — 19 Academy Award nominations, the most of any actress in history, and three wins — her friends and fellow actors most admire Streep for her devotion to Cazale, for the strength of character such a young woman showed.

“She took care of him like there was nobody else on Earth,” said Joe Papp. “She never betrayed him in his presence or out of his presence. Never betrayed any notion that he would not survive.”

Al Pacino agreed.

“When I saw that girl there with him like that I thought, ‘There’s nothing like that.’ I mean, that’s it for me. As great as she is in all her work, that’s what I think of when I think of her.”