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The "Rocky" and "Terminator" actors had talked about uniting on screen for 20 years, Schwarzenegger says, but never found the right script at a time when they were both free of other commitments.
"There was always something off. The script was not right, or the studio was not as much interested in it as we were. ... It just didn't happen," Schwarzenegger said at a Television Critics Association meeting, where he appeared to promote an upcoming documentary on ESPN website Grantland.com.
"Then one day I was sitting at my hairdresser, and Sly came up to me and he said 'oh man, it would be so great if you could be doing just a little bit of something in my 'Expendables' movie," said the 65-year old actor-turned politician.
At the time, Schwarzenegger was serving as governor of California and had put his action movie career on hold. He said he agreed right there to film a few hours for "Expendables" on weekends so it wouldn't take time away from his gubernatorial duties, and that he worked for free. The movie came out in 2010.
When Schwarzenegger's cameo was well-received, Stallone approached him to appear in "The Expendables 2." Schwarzenegger, who stepped down as California governor in January 2011, said he did four days of work in Bulgaria on the film, which reaches US theatres this month.
Their next project came together when Stallone told the producer of upcoming film "The Tomb" that he didn't want to make the film unless Schwarzenegger was involved. Schwarzenegger liked the script and joined the film - one of five movies he has in the pipeline.
While joking that the two were "in love with each other," he said they have no plans to work on other films together. But he added: "I'm sure 'Expendables 2' is going to be a big hit. ... Then (Stallone) will be probably coming back again and asking me to be in the next movie."
In another return to the screen, Schwarzenegger will appear in a documentary short film called "Arnold's Blueprint" that focuses on the former bodybuilder's service in the Austrian Army.
Schwarzenegger said his time in the military as a teenager helped shape his future career as a bodybuilder, actor and politician. The story "is about overcoming obstacles," he said.
Schwarzenegger said he worked around his military training to find time for bodybuilding. He later won bodybuilding titles, and used the sport to propel him to international stardom.
"I had a very clear vision of where I wanted to go," he said. "You realize you have to pay no attention to the naysayers. When you learn those lessons in sports, you can apply those lessons for the rest of your life," he said.
The short film will premiere on ESPN's Grantland.com website on September 26 as part of the network series "30 for 30 Shorts."
Schwarzenegger announced earlier this week he was establishing a global policy think-tank in his name at the University of Southern California that would examine non-partisan politics.