The actor explained that his relationship with Stallone started off as professional since Stallone was his boss, but they developed a friendship.

Sylvester Stallone's historic "Rocky" story will be retold in Peter Farrelly's biographical drama, "I Play Rocky." In order to bring it to life, Farrelly had to flash back to how "Rocky" became a household name, and it started with Stallone selling his screenplay only if he was cast as the lead role.

In the "I Play Rocky" trailer, Stallone – who is played by Anthony Ippolito – presents the script that he wrote in three days to movie producers. They offered him outrageous amounts of money for the script, but Stallone wouldn't budge. Eventually, the producers gave in and cast Stallone in the role that changed his life.

Sylvester Stallone wrote "Rocky" in three days and refused to sell his screenplay unless he was cast as the lead role. (Getty Images)

"It was unheard of at the time, especially for someone in Stallone's economic situation which was dire," film historian Dwight Cleveland said. In the trailer, Stallone was seen working as a valet attendant when he created "Rocky."

SYLVESTER STALLONE REVEALS GRUELING ‘ROCKY’ PREP: ‘IF YOU WANT IT TO FEEL REAL, YOU HAVE TO LIVE IT’

Tisha Morris, entertainment attorney of Legacy Arts Law, said: "Stallone’s decision was extraordinarily unusual. Most first-time writers would have been expected to take the money and relinquish casting control."

From a legal standpoint, Stallone had every right to pull the move he did.

"Legally, Stallone could retain the screenplay and impose that condition because he owned the copyright," she continued. "However, the risk was whether the producers would simply walk away, leaving him with neither a sale nor a film. He was effectively betting the screenplay’s entire market value on himself as an actor."

Cleveland said that Stallone acted just like his character in the movies when it came to the initial negotiations.

"Just like his character, Stallone had the confidence his story was a true winner, so he held his ground knowing that this was his shot at stardom both as an actor and as an author," he said. "He wasn't willing to give that up to someone else and be left without any clout in Hollywood. Film depicts the human condition and in this case it was true to the heart."

Talia Shire and Sylvester Stallone appear on a poster for "Rocky" in 1976. (Getty Images)

"Stallone was an unknown actor with only minor bit parts to his name, and producers Robert Chartoff and Irwin Winkler wanted to buy his script outright — with reports indicating the offers escalated to $360K with stars (such as Ryan O'Neal, Burt Reynolds, and James Caan) identified for the lead role," Simon said.

He continued, "In monetary terms today, it’s probably the equivalent of someone being offered over $2M. Stallone bet everything on starring in the film — a role that no one wanted him to play. He owned the property, and put it all on the line for his million-to-one shot, and like Rocky himself left an indelible mark on cinema and culture, which will be an everlasting legacy."

Beatrice Straight and Sylvester Stallone at the 49th Academy Awards in 1977. (Getty Images)

According to Simon, it was a win-win for producers to ultimately agree to Stallone's terms.

"Financially, Stallone's insistence on starring came paired with his willingness to accept a dramatically reduced salary and rights fee, which lowered the film's overall budget risk at a time when 'Rocky' was made on a modest budget of roughly $1 million," he said. "So, the economics became more palatable to a studio like United Artists that was otherwise wary of an unknown actor carrying the film."

Released in 1976, "Rocky" became one of the biggest underdog success stories in Hollywood history. The movie follows small-time Philadelphia boxer Rocky Balboa as he gets the once-in-a-lifetime chance to fight heavyweight champion Apollo Creed.

Sylvester Stallone on the set of his movie, "Rocky II." (Sunset Boulevard/Corbis via Getty Images)

Made on a modest budget, Rocky was both a critical and commercial hit, earning 10 Academy Award nominations (including best actor and best original screenplay for Stallone) and winning three Oscars: best picture, best director for John G. Avildsen and best film editing.

"Rocky" went on to launch one of cinema's most successful franchises and cemented Rocky Balboa as one of the most iconic characters in movie history.

Sylvester Stallone became a household name thanks to the "Rocky" franchise. (Getty Images)

Now, Amazon is gearing up to release "I Play Rocky" in November. According to reports, Stallone is not involved in the biopic. Speaking to The Playlist, Stallone said, "I was shocked to read [about] it. I have zero to do with it."

Ippolito is set to star as Stallone and AnnaSophia Robb will play Sasha Czack, Stallone's first wife. Other members of the cast include: Stephan James, Matt Dillon and P.J. Byrne.

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