Arnold Schwarzenegger has made a whole career on three words first uttered in 1984's The Terminator: "I'll be back." So it's hardly surprising that he's returned to his iconic signature role in this summer's Terminator Genisys.

Arnold Schwarzenegger in 'Terminator: Genisys'Arnold Schwarzenegger felt young after getting into shape for 'Genisys'

"Well, I always said that I would do another Terminator if the story is great," he says. "We saw the original come out, and James Cameron outdid himself with the sequel. Since then, we've been trying to outdo that, not always successfully, but there is always the attempt. This time I think we've done an extraordinary job."

More: Read our review of 'Terminator: Genisys'

Of course, this meant getting back into prime physical shape at age 67. "It's like I've been doing it all my life," he laughs, "because I'm very passionate about the character. I trained twice as hard, gained muscle size to keep the same frame, and wear the same size clothing. I myself don't feel any older because I stay in shape and I exercise every day, so when I started the movie I did the prepping and exercised and trained for it so I could do the movie and all the stunts necessary for it. I felt in great shape and really young."

Watch the trailer for 'Terminator: Genisys' here:

And this time he gets to battle his younger 1984 self, played on-set by a champion bodybuilder stand-in. "I think the bodybuilder they picked for me to fight with was really extraordinary," Schwarzenegger says. "Even after three or four days of doing this fight scene and being thrown around and doing all the crazy stunts of this epic battle, I was always wondering how are they going to be doing it. How are they going to do all these crazy cuts and head replacement and how do you make the body exactly like my body? I looked very carefully when I watched it the first time and I just thought it was so seamless."

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Schwarzenegger believes the key to a long and varied career is to never take yourself seriously. "To me, life is all about hard work. My father always said to me, 'Be useful.' To be useful, it's very important to do something that is bigger than you are, causes that are bigger than you are, but have a good time at the same time. Smile, have a good time, have a positive attitude and see the glass as half-full rather than half-empty."