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The Expendables co-stars Sylvester Stallone and Arnold Schwarzenegger are re-teaming yet again, this time for the action-thriller The Tomb, and ET is first on the set!

"We're just a couple of old war horses talking about the oddest subjects on the planet," Sly tells Christina McLarty. "I think in the beginning we were competitive -- there's no question about it. A little alpha territorial. And then we realized it was good for our careers [to work together]."

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"The funny thing is we have tried, I think, our entire careers to always work together," says Arnold. "We have talked about it for three decades and it never happened. I think we tried maybe too hard. … But now after the governorship, somehow this just fell into place."

Arnold explains that he worked four hours on the first Expendables, and then expanded to four days on The Expendables 2, out August 17. "We worked really well together," he says, encouraging more collaborations.

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In Summit's The Tomb, out in 2013, Sly plays "the world's foremost authority on structural security" for high-security prisons who finds himself faced with escaping from the master prison of his own design. The former Governator plays "a complex inmate with multiple shades of grey" who fights to keep the prisoners from losing their humanity in their darkest hour as they struggle together to stay alive.

"This genre unfortunately is becoming… let's just say it's fading away," says Sly. "You have the superheroes today which are possessed with all extraordinary powers; they can blink and a fireball comes out of there. It's great. And then you have a bunch of us which is just your basic male-pattern badness. … Kind of like hands-on action."

Watch ET for more with Sly and Arnold!