The 63-year-old actress says she lost too much weight while prepping to reprise her role as Sarah Connor.
"There were no carbs in my life for a year. I got so lean, when I arrived they had to build me a butt," Hamilton admitted while appearing on ITV's "Lorraine" Wednesday.
"They added bottom, something you don’t ever see coming -- who wants more bottom? -- but apparently ... I shouldn’t give my secrets away!" Hamilton continued.
She also explained what type of training she was required to do: "Time off was military training, Spanish lessons, scuba, we had to do it all. There was never a moment we weren’t learning something or acting."
Hamilton joked that the films are getting so intense she's threatened to fake her own death not to star in them.
"I've been threatening to fake my own death so I wouldn’t have to do the next one if there is one," she admitted. "It really was hard, it was the hardest and the greatest I’ve ever done."
Hamilton's co-star, franchise newcomer Mackenzie Davis, agreed with Hamilton about the intensity of the training. "It was so hard making this god damn movie," she said.
Hamilton joked that the women are tougher than Arnold Schwarzenegger, who also returned to play the futuristic half human-half robot.
"We get to do some really good work together this time rather than just running from him," she said of reuniting with the action hero.
"Terminator: Dark Fate" hits theaters on Nov. 1.