The 'Tulsa King' star recently opened up to ET about his legacy and career resurgence.
Sylvester Stallone has a lot to be proud of over the course of his career, as well as some major regrets -- but ultimately, the actor says he has learned some profound lessons from his children that shaped him as an actor and as a human being.
Over the weekend, Stallone walked the carpet at the Toronto International Film Festival premiere of the new documentary, Sly -- which details his personal and professional life -- and he spoke with ET's Rachel Smith about the raw and candid examination of his career seen in the film, helmed by documentarian Thom Zimny.
"We spent a couple years together and I just let my guard down," Stallone said of working with Zimny. "I was speaking, quite frankly, like if you only have a limited amount of time on Earth, and you wanted to say things, and you didn't really guard yourself, and just let it go."
The film deals with Stallone's experiences with an abusive father and his late son Sage and paints a fuller portrait of the man behind the massive Rocky franchises.
"I've seen a lot of documentaries and most of them talk about your body of work, and this one here talks about why you are the way you are," Stallone said. "Like, who molded you, from what environment, and this is what molds everyone."
"We don't ever escape our childhood and certain things are profound, they mark you, and they mold you in the way you perceive life," he said. "And that's what this is about."
Stallone is the father of five children, including sons Sage -- who died in 2012 at age 47 -- and Seargeoh, from his first marriage to Sasha Czack. He also shares three daughters -- Sophia, 27, Sistine, 25, and Scarlet, 21 -- with wife Jennifer Flavin.
According to Stallone, his children led to him coming to his biggest realization, which is that he wasted too much time on other things before really prioritizing his family.
"Every actor feels the time crushing in on them, and eventually it'll all go away, it's gonna dissipate. No one remains on top forever. There's a valley," he said. "So you fight so hard to keep your career going and get that finite script that's gonna define your career -- but you do it at a great cost. And usually your family is the one that suffers this kind of forced abandonment."
"Then, as your career begins to weigh in, you're like, 'Oh my God, I've lost the real movie!' Which is the movie of your life, and these are the characters that play the most profound roles in your life," he added.
The film also raised the question of Stallone's legacy, and how he serves as a guardian to it as his career goes on. However, Stallone said that his view on his own legacy has changed dramatically over the course of his life.
"It's really easy to say, 'Never give up.' [But] that's hard to do," Stallone said, advising other actors to "learn to deal with failure."
"That's the thing! Because there's always the chance, always, for the comeback," he added. "So my legacy is all about the comeback."
It also seems to be about building bridges and forging friendships. One element of Sly that plays a big role -- and has played a role in Stallone's career in many ways -- was his professional rivalry and occasional animosity with now-friend Arnold Schwarzenegger. In the early days, during the height of their fame, both stars were certified action heroes and would often be in the running for the same roles, which only served to intensify their quasi-feud.
According to Stallone, Schwarzenegger used to be "the most competitive" -- but with time, things changed.
"Now I understand the fella," Stallone said with a laugh.
Stallone and Schwarzenegger have now become close friends, and have walked similar paths when it comes to a career resurgence. Both Stallone and Schwarzenegger are both starring in TV shows -- Tulsa King and FUBAR respectively -- and both are the subject of retrospective documentaries on N Netflix.
Fans can see more about Stallone's rivalry -- as well as how his family and his past shaped his perspective and his legacy when Sly premieres Nov. 13 on Netflix.