James Gandolfini, who brought a balance of intensity and vulnerability to his portrayal of conflicted mob boss Tony Soprano that made him among the most respected actors of his generation, died Wednesday in Rome. He was 51 and was believed to have suffered a heart attack.

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According to the Taormina Film Festival, he was on his way to the film festival where he was expected Thursday. He had been expected to participate in an onstage conversation with Italian director Gabriele Muccino on Saturday at the Sicilian festival.

Gandolfini’s commanding screen presence was the driving force in establishing “The Sopranos” as the most influential TV show of the past generation. The actor was praised for his deft juggling of the character’s violence and sensitivity, making the murderous crime lord a sympathetic figure that set the mold for the flawed anti-heroes that populate cable dramas today. Underscoring the show’s continuing influence, “Sopranos” was voted the best-written series of all time in a recent Writers Guild of America survey.

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Gandolfini had an active career in movies, TV and on stage before he inhabited Tony Soprano. But it was the role created by David Chase of the New Jersey mob boss who decides to see a psychiatrist to deal with his emotional issues that catapulted him into mega-stardom. Balding and beefy, Gandolfini was not conventionally handsome but became a sex symbol through the show’s immense popularity.

“He was a genius. Anyone who saw him even in the smallest of his performances knows that. He is one of the greatest actors of this or any time,” Chase said in a statement. “A great deal of that genius resided in those sad eyes. I remember telling him many times, ‘You don’t get it. You’re like Mozart.’ There would be silence at the other end of the phone. For (wife) Deborah and (children) Michael and Liliana this is crushing. And it’s bad for the rest of the world. He wasn’t easy sometimes. But he was my partner, he was my brother in ways I can’t explain and never will be able to explain.”

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Gandolfini later acknowledged that he was surprised to have landed the lead role, given his history as a supporting player. Others considered for Tony Soprano include Anthony LaPaglia. But Chase responded to Gandolfini’s naturalistic style and the authenticity he brought as a native son of New Jersey. Like the character, Gandolfini was known to have a prodigious appetite for eating and drinking.

Writer-director Steve Zaillian was an intimate of Gandolfini’s for years, working with the thesp before and after the “Sopranos” era. He directed Gandolfini in the 1998 drama “A Civil Action” and more recently the two pacted to produce a crime drama pilot for HBO, “Criminal Justice,” in which Gandolfini also appeared.

Zaillian noted that the “Sopranos” success never changed his old friend. He described Gandolfini as “honest, humble, loyal, complicated, as grateful for his success as he was unaffected by it, as respectful as he was respected, as generous as he was gifted. He was big, but even bigger-hearted.”

“I thought that they would hire some good-looking guy, not George Clooney but some Italian George Clooney, and that would be that,” Gandolfini said in Vanity Fair’s extensive oral history of “Sopranos” published in April 2012.

The suddenness of Gandolfini’s death devastated the “Sopranos” extended family of actors, creatives and executives at HBO. The show remains a staple of HBO’s suite of channels, and the cabler was scrambling Wednesday evening to put a tribute slate in front of episodes already scheduled to air.

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“We’re all in shock and feeling immeasurable sadness at the loss of a beloved member of our family. He was special man, a great talent, but more importantly a gentle and loving person who treated everyone no matter their title or position with equal respect. He touched so many of us over the years with his humor, his warmth and his humility,” HBO said in a statement.

“Jimmy was the spiritual core of our ‘Sopranos’ family, and I am stunned at this devastating loss. He was a great talent, but an even better man,” said Starz CEO Chris Albrecht, who greenlit “Sopranos” in his previous role as head of programming and CEO of HBO.

“Jimmy was one of the most talented, authentic and vulnerable actors of our time. He was unorthodox and truly special in so many ways. He had the sex appeal of Steve McQueen or Brando in his prime as well as the comedic genius of Jackie Gleason. I’m proud to have been his friend and grateful for the extraordinary years I was lucky enough to work with him,” said Paramount chairman Brad Grey, who produced “Sopranos” through his Brillstein-Grey Television banner.

Chase’s script for “Sopranos” famously bounced around Hollywood in development for years before landing at HBO. But it took an actor of Gandolfini’s talent to breathe life into his character, particularly in the scenes depicting his one-on-one therapy sessions with the counselor Jennifer Melfi played by Lorraine Bracco.

“If you took the Melfi scenes away, you wouldn’t care about this man as much, or care about anything that was happening to him,” Gandolfini told Vanity Fair.

“We lost a giant today. I am utterly heartbroken,” Bracco said.

As much as “Sopranos” is remembered for its organized crime storylines, the show at heart revolves around Tony Soprano’s conflicts with his wife and children. His work with Edie Falco, who played Carmela Soprano, was routinely hailed for the honesty and intensity the pair brought to the screen. Like so many other cast members, Falco emphasized how much she admired Gandolfini as an actor and and as a person.

“He was a man of tremendous depth and sensitivity, with a kindness and generosity beyond words. I consider myself very lucky to have spent 10 years as his close colleague,” Falco said. “The love between Tony and Carmela was one of the greatest I’ve ever known.”

After “Sopranos” ended its six-season run in 2007, Gandolfini limned a range of supporting roles in such pics as “Zero Dark Thirty,” “The Incredible Burt Wonderstone,” comedy “In the Loop” and Chase’s period rock ‘n’ roll drama “Not Fade Away.”

He had been working on Fox Searchlight’s “Animal Rescue,” now in post-production, as well as the HBO limited series “Criminal Justice” and the CBS comedy “Taxi 22.” He served as exec producer on docus “Alive Day Memories: Home from Iraq” and “Wartorn: 1861-2010,” as well as on HBO’s miniseries “Hemingway & Gellhorn.” He developed “Criminal Justice” and numerous other projects as a producer through his Attaboy Films banner.

“It was such an honor to work with Mr. Gandolfini and nothing but a pleasure to see him perform. He was one of the greatest. I wanted to make him proud with the movie we made together and now it will be in his loving memory,” said “Animal Rescue” director Michael Roskam.

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Gandolfini generally kept out of the spotlight and rarely courted media attention. But he was a vocal advocate of wounded military veterans, and used his fame to draw attention to the 2007 docu “Alive Day Memories,” which recounted incredible stories of survival from soldiers in Iraq and Afghanistan.

In 2011, he drew praise for his role as a pioneering reality TV producer in HBO’s Loud family telefilm “Cinema Verite,” directed by Robert Pulcini and Shari Springer Berman.

“Jim was an icon, and anyone who had the pleasure to spend ten minutes with the man understood why. His talent was bigger than life, and so was his generosity to both directors and fellow actors. We’re devastated for his family, and heartbroken that we’ll never get a chance to work with him again,” Pulcini and Berman said.

Gandolfini had six Emmy lead drama actor nominations as well as a Golden Globe for his work in the “Sopranos.” He first broke through in movies as hit man Virgil in “True Romance,” and went on to appear in films including “Mr. Wonderful,” “The Man Who Wasn’t There” and “Get Shorty.” Among his other film roles were “Crimson Tide,” “Night Falls on Manhattan,” “She’s So Lovely” and “The Taking of Pelham 1 2 3.”

“Our hearts are shattered and we will miss him deeply. He and his family were part of our family for many years and we are all grieving,” said longtime Gandolfini managers Mark Armstrong and Nancy Sanders.

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A native of Westwood, New Jersey, his mother was born in the U.S. but raised in Italy and his father was born in Borgotaro, Italy; his parents spoke to him in Italian at home. He attended Rutgers U. and became interested in acting when he accompanied his friend Roger Bart to an acting class.

In 1992, he starred in “On the Waterfront” on Broadway for six months, and returned to the stage in 2009 in “God of Carnage.”

He is survived by his wife, Deborah Lin; two sisters; a son and a young daughter.

(Pat Saperstein and Nick Vivarelli contributed to this report.)