George Pimentel / WireImage 1 / 15
George Pimentel / WireImage 1 / 15

"Shocking" is easily the best adjective to describe Matthew McConaughey's physical transformation in the upcoming drama "Dallas Buyers Club." But it's not a word that the 43-year-old actor would use to describe his children's reaction after witnessing their father unload nearly 50 pounds to play HIV patient Ron Woodroof in the biopic.

"They remember that time, 'Remember when dad was so skinny?'" recalls McConaughey, referring to his three kids with wife, Camila Alves. "They remember that if you ask them now, but there was no shock because they were living with me. … It was people that I ran into that I hadn't seen in months that got shocked."

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One of those people was Jennifer Garner, who has a supporting role in the well-reviewed film as Dr. Eve Saks, a compassionate physician who supports McConaughey's character as he battles the medical establishment over alternative treatments after being diagnosed with HIV in 1986. Garner, who co-starred with McConaughey in the 2009 romantic comedy "Ghosts of Girlfriends Past," says her reaction was much more emotional.

"The first time I saw Matthew was at a hair and makeup test," remembers the mother of three. "I walked in and I talked to him normally, and then I went into another room where he couldn't see me and I took a deep breath … because it made me just shake. It felt like a friend of mine was really, really ill. You can't look at somebody who is that emaciated and not have that reaction. And then from there, the next five weeks, he lost maybe 12 more pounds, and it just got more and more upsetting."

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Hearing McConaughey detail his weight loss is much easier on the ears. The actor claims the hardest part of his journey from 182 pounds to 135 pounds was the first few days. "The hardest part mentally ... was just at the inception. It was like two days and then it was easy," reveals the actor, who is already generating Oscar buzz for his performance in the film, scheduled for release on Nov. 1. "Physically, I just lost all leverage."

It wasn't all loss, however. McConaughey said his brain received a major boost. "What happened though, which was really cool and unexpected, all of the energy -- say I lost 40 percent energy from my neck down, that 40 percent got added to the neck up. My brain became clinically sharp. ... I was just ... like a surgeon," he said.

And Garner agrees. "He really was," she said. "You could say, Matthew, what is the square root of 400,568, and he would say, 'ba, da, da,' and he would know. What's the capital of Kazakhstan? He was sharp. He was really on top of his game."

Speaking of the top, Garner also gushed about the performances of McConaughey and Jared Leto, the latter of whom also transformed himself physically to play a HIV-positive transvestite who becomes an unlikely friend to McConaughey's character. "I had the best seat in the house," Garner said. "Their performances transcend the newsy bit of them losing all the weight."

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One more newsy bit? Garner added that she, too, has transformed herself. But not in the same way the men have. "I've lost overall about 140 pounds over times from having three kids and losing all the weight. It's just that nobody really celebrates that," she said.