Sylvester Stallone on Sage's Death: It's Very Very Tough'

Prior to the London premier of the film, the four actors spoke with ABC News' correspondent Cecilia Vega.

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Even though these co-stars are violent with each other on-screen, off-screen they participate in friendly banter and enjoy working with one another, according to Stallone. The actor, who co-wrote the movie, said that when he asked everyone to do the sequel, they all sounded enthusiastic and jumped at the opportunity.

Who could blame them? "The Expendables" brought in almost $300 million at the box office. 

"It was great to be back because the spotlight was not on me," Schwarzenegger said. "I was very appreciative that Sly was interested in having me in the movie."

Keeping busy with work has also helped Stallone keep away depression, he said. Tuesday's "Good Morning America" was the first time Stallone had spoken about his 36-year-old son's death. 

Sage's body was discovered July 13 in his Los Angeles home.

"It's very, very tough," Stallone said in the "GMA" interview. "It's a horrible situation, but time hopefully will heal, and you try to get through it, but it's just something that's a reality of life." 

Stallone said he evades depression by keeping busy with work. 

"It's important to get back and try to start reliving your life, otherwise you can go into a spiral," he said. 

Toxicology tests will determine the cause of Sage's death, according to People magazine. However, according to a previous Enstarz report, Sage's mother, Sasha Czack, said her son had five teeth removed during one procedure only two weeks before his death.

"I told him not to do that. I've heard about people dying having multiple procedures done to your mouth. Do not have more than one tooth (pulled)," she said.

Czack also admitted that her son had taken medication to combat the pain he was experiencing.

Stallone hasn't been able to escape the public eye. His nephew Edd Filiti blamed him for the cause of Sage's death. According to Facebook comments obtained by Celebuzz, Filiti wrote: "Neither you [Stallone's wife Jennifer Flavin] or he could return a phone call, which is all he wanted, his father. I know you don't care what happened, but you'll care when your (sic) squirming under the burning magnifying glass of public opinion."

According to a Celebuzz source, Sage was "depressed" prior to his death because Stallone had not returned his phone calls where he wished his dad a happy 66^th birthday.

"Sage was not allowed in his father's home and he admitted to other family members of his devastation that he'd seemingly been cut out of his father's life," the source said.