In the latest issue of Closer Weekly, those close to Paul Newman and Robert Redford open up about their explosive on-screen chemistry and even stronger real-life bond.

Their 40-year friendship began in 1968, when big-screen icon Paul fought to get lesser-known Robert cast as his partner in crime in Butch Cassidy and the Sundance Kid.

RELATED: Is Robert Redford Retiring from Acting?

“We developed a friendship off that film, and things fell into place,” Robert recalled during a recent TimesTalks Q&A earlier this month. “It didn’t require a lot of talk but what came with it was fun.”

Robert also admitted during the Oct. 8th Q&A, “I owe much of my career to Paul.”

robert redford and paul newman

Robert (left) and Paul (right) in Butch Cassidy and the Sundance Kid. (Photo Credit: Getty Images)

That unshakable connection extended beyond Butch Cassidy and the Sundance Kid to 1973’s Oscar-winning smash The Sting — and into their private lives, as they supported each other through marriages, births, illnesses and deaths.

“They were best friends,” their The Sting co-star Sally Kirkland tells Closer. “They really loved each other, and that was so great and so contagious to be around."

RELATED: 'He Has Gone Through Hell' — Inside Robert Redford's Secret Past of Unspeakable Tragedy

Call it Hollywood’s first bromance. Never had two major matinee idols teamed up to such adoration; there was no Jerry Lewis or Lou Costello in this combo. “If two guys look like Adonis in a movie, nobody is Adonis,” says Shawn Levy, author of Paul Newman: A Life. “They can let their hair and their guard down and be more natural — there’s a playfulness to their collaboration.”

This pair of blue-eyed golden boys made it look effortless. “They both had this feeling like, I thought I was the most beautiful person in the world, then you come along and shake me out of it, so I’m going to tease the hell out of you,” says David Thomson, author of The New Biographical Dictionary of Film.

robert redford and paul newman

Robert (left) and Paul (right) in October 1969.

It wasn’t an act: Robert and Paul really did feel like blood relatives. “The guys were so close, Paul’s kids used to refer to Robert as Uncle Bobby,” an insider reveals to Closer. “They secretly hoped one of Paul’s daughters would marry Robert’s son.”

“People admire them both for not being ‘Hollywood guys,’” says Levy of Paul, who lived in Connecticut, and Robert, who spends much of his time in Utah and New Mexico. “They’ve always been more than just movie stars.”

RELATED: Paul Newman — Inside His Dangerous Passion

The pair enjoyed more trivial pursuits as well, like playing pranks on each other and driving fast cars. “During The Sting, they bought matching silver Porsches,” producer Michael Phillips tells Closer. “They would race each other up the Pacific Coast Highway. We were all concerned, but Paul was a professional driver — he knew what he was doing.”

At the TimesTalks Q&A, Robert said he supported Paul in his decision to pass on re-teaming for A Walk in the Woods. "As he was getting older, he couldn’t physically do what he used to be able to do, so being Paul, being an honest man, with a lot of integrity and a lot of generosity, he said, 'Rob, I don't think I can do this,' so that made me sad but I had to accept what he said but we still maintained our friendship."

robert redford and paul newman

Paul (left) and Robert (right) in October 2004.

In the months before his Paul's death from lung cancer, “Robert visited him a few times, but toward the end, Paul only wanted to see immediate family,” a friend tells Closer. “He always prided himself on being fit, healthy and happy, and he didn’t want longtime friends to see how he’d deteriorated.” The pals chatted on the phone, “but when he’d hang up, Robert would be heartbroken because he could hear his good friend losing his battle.”

RELATED: Joanne Woodward is Losing Memories of Late Husband Paul Newman as She Suffers from Alzheimer's Disease

When Paul’s struggle ended in 2008, “Robert was totally devastated,” says the friend. “It was as if a part of him died.”

“Robert was there for [Paul’s wife] Joanne Woodward after [his] death — he checked in on her and the kids often,” the friend tells Closer. Though Joanne has tragically lost her memory due to Alzheimer’s disease, “Robert remains in touch with the kids and is always there for them.”

For more on the friendship of Robert Redford and Paul Newman, pick up the latest issue of Closer — on newsstands now!