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One spring day in 1956, Harry Belafonte picked up the phone and heard the voice of Martin Luther King Jr.

"You don't know me …" King said.

"Oh, I know you," Belafonte replied, as he recounted later in his memoir. "Everybody knows you."

King seemed apprehensive, as Belafonte told me in a 2017 interview, because "he didn't know where he was headed" and "his mission was not that clear."

Belafonte's mission was not that clear either, not in the spring of 1956. Around that time, he released a new album, "Calypso," which would sell more than a million copies and propel his Hollywood acting career.

But the call between the two men changed the course of both their lives. And it changed the way that celebrities would think about using, and imperiling, their stardom to advance a social cause.

Just months earlier, Rosa Parks had refused to give up her seat on a segregated bus in Montgomery, Ala., sparking a citywide boycott and thrusting King into a position of leadership that neither he nor anyone else could have imagined. Now Belafonte was cast into a new role, too, one that caused him to suppress his career ambitions as he joined King in pursuit of loftier goals.

King sought to build a support system in those early days and sensed "that I was one of the forces in that historical moment that was willing," as Belafonte later told me, "to commit to that cause."

But Belafonte might not have committed so fully if not for the almost instant bond the men forged. Even in that first phone call, Belafonte said, he was struck by King's attentiveness. King listened more than he spoke. He asked questions. He keyed in on Belafonte's interest in labor unions, wondering how unions might support this growing movement, which, it should be remembered, had not yet transcended Montgomery.

"In the very beginning, Dr. King was not quite as vocal as he became," Belafonte said. "He was very much a listener. He was in a turf with which he was not deeply familiar." Though many people received King warmly, Belafonte said, "he just did not know where he fit. His task was to gather around him people who could help reveal for him what his course would be."

Belafonte, who died Tuesday at 96, would prove to be one of his most important guides.

Even in the earliest days of the Montgomery bus boycott, King observed that the northern news media would play a major role in shaping national opinion and driving political change. Reporters couldn't get enough of the Baptist preacher with an advanced degree in theology. King captivated television and radio audiences. Even among Baptist preachers, King's spellbinding oratory and charisma were rare.

But they were not so rare among Hollywood stars, as both he and Belafonte recognized.

Belafonte, working in connection with King, became the key strategist of a group of celebrity activists that included Ossie Davis, Ruby Dee, Sammy Davis Jr., Dick Gregory and Sidney Poitier. These and other celebrities risked their careers by joining the fight for racial justice and helped redefine what we expect of celebrities today.

Belafonte passed up opportunities to make films in favor of his work as an activist. Even after he became the first Black person to win an Emmy in 1960, further deals to do more television dissolved when a sponsor, he has said, balked at a show potentially featuring Black and white performers in collaboration.

But what struck me most strongly as I listened to Belafonte, over the course of an afternoon in his apartment in Manhattan, was not his bravery or his commitment to the cause. I was struck more by his personal commitment to King.

Over the past six years, I've interviewed dozens of people who knew King, but few of them showed the kind of care for King's well-being that Belafonte did.

When King visited New York, as he did many times, he stayed at Belafonte's home. Late at night, sometimes, the men would sit alone in Belafonte's den and listen to music. King would take off his shoes and socks and sing along to the music of Lead Belly, Odetta and Peter, Paul and Mary.

Once, Belafonte said, he noticed that his friend had developed a kind of nervous tic, a frequent hiccup so soft one had to be standing beside him to hear it. When the hiccup suddenly vanished, Belafonte asked how he had cured it.

"I made peace," King told him.

"With what?" Belafonte asked.

"With death," he said.

Belafonte remained modest about his role in King's life. "I don't want to anoint myself," he said.

He didn't have to.

It was Belafonte who tried to relieve some of the strain in King's marriage by hiring housekeeping help for his family. It was Belafonte who put away money for the education of the King children. It was Belafonte who purchased a life insurance policy for his friend. And it was Belafonte, time and again, who raised emergency funds for King's movement, especially when money was needed to post bond for jailed protesters.

His activism didn't end with King's death. King was two years younger than Belafonte. When we pause to reflect on what King might have done with the gift of a long life, Belafonte offers us a splendid vision.

With his connection to King, Belafonte both exploited his newfound fame and, when necessary, subsumed it. He showed how to use a personal spotlight to bring attention to a cause — and also how to shift that spotlight to the person best at delivering the message.

For the rest of his life, Belafonte continued refining that blueprint for celebrity activism. He never tempered his radicalism. He never quit the fight. Like King, he never stopped speaking truth to power.

During our conversation, he complained that American history did students a disservice by ignoring its radical figures. We choose a version of history that makes people comfortable, he said. As a result, we do nothing to encourage the kind of bold behavior that he and King and many others exhibited.

King made people uncomfortable and "put Blackness squarely in everybody's space," Belafonte said.

And Belafonte, with his raspy, courageous voice, did the same, for another 55 years.

Jonathan Eig is the author of "King: A Life," a forthcoming biography of the Rev. Martin Luther King Jr. This article originally appeared in the New York Times.