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• Sylvester Stallone recalled making "Rocky" in Netflix's new documentary, "Sly."
• The star thought the 1976 movie would fail because most of an early audience left after 20 minutes.
• "Rocky" spawned five sequels and the "Creed" spinoff movies.

In Netflix's new documentary, "Sly," Sylvester Stallone admitted that he thought "Rocky" would bomb when ¾ of an early screening walked out after the first 20 minutes.

Released in 1976, "Rocky" is one of the most iconic boxing movies of all time, making $117 million at the US box office, and even winning best picture at the Oscars. Stallone returned to the ring for five sequels and later reprised the role in 2015 in the "Creed" spinoff movie starring Michael B Jordan.

In the documentary, Stallone recalled that before opening in theaters, "Rocky" screened at the same theater that he used to work at when he was younger.

"The first review we got out of New York by Vincent Canby was scathing, so I didn't know what to expect," Stallone said. "We screen 'Rocky' five days before release. It was an afternoon matinee."

Stallone's brother, Frank, added: "I was sitting there with him, and all of a sudden he's going, 'Oh, shit.' Talk about a crestfallen look. He said, 'Oh, god.' He thought, 'Man this is going to be a bomb.'"

Stallone went on to say: "20 minutes into it, the audience, ¾ were gone. And I'm getting lower in the seat. I'm going, 'Oh, shit.' So when it did open, my confidence was not soaring."

The actor said that the audience members that did stay in their seats soon became engrossed in the film, particularly when Rocky fights Apollo Creed (Carl Weathers).

"When everyone's inside, I go to the back, and I'm standing there. It's almost like it's a still painting. No one is moving, everyone is listening to every word. Like, boom. Riveted," he said.

"When he knocks Apollo down, whole theater went up," Stallone added. "It's like, holy shit, the audience is participating like it's a real sporting event. We blurred the lines," he added.

"Sly" is now streaming on Netflix.

Disclosure: Mathias Döpfner, CEO of Business Insider's parent company, Axel Springer, is a Netflix board member.

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