ET's Kevin Frazier spoke with Garner on Friday, while promoting her upcoming thriller, where they were joined by Duggins who dished on the film's intense fight sequences and her bond with Garner.
The actress said that Duggins, who is one of her "closest friends," pushed her to get into the headspace she needed to throw a punch or shoot a gun with a fiery vengeance. As for Duggins, she's learned how to get into Garner's headspace when she needs to double for her.
"I just become her at times," said Duggins, who added that Garner really sets the bar high when it comes to being a warrior. "Come on! Just look at the action and how badass she is!"
The pair also gave Frazier a bit of a demonstration in stage fighting, with Duggins and Garner both showing off their moves.
While Duggins has been in the screen fighting game for years, she had nothing but praise for Garner's skills and determination. "It's her the whole movie," she said. "You see it on her face the whole time."
"But she would come in and fix me and I would do it again and then I would look closer to what she did," Garner said, referring to the movie's brutal combat scenes. "But I'll never be like her… She just always takes anything and it's just elevated when she does it."
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'Peppermint' Trailer: Jennifer Garner Transforms Into a Vigilante Hero
In Peppermint, Garner plays a woman whose husband and daughter are murdered in a drive-by. When she awakes from her coma, she dedicates her life to learning how to fight and kill, then seeks out those who took everything from her.
"This is hard R action. This is not a family movie or Miracles From Heaven," Garner told Frazier, referring to her 2016 religion-themed drama.
While Garner has mostly appeared in grounded dramas and family comedies in recent years, she's certainly no stranger to action -- including her turn as Elektra in 2003's Daredevil and 2005's Elektra, and her five-season run on J.J. Abrams' ABC spy thriller Alias, which came to an end in 2006.
"My stunt double and I have worked together since I think the third or fourth episode of Alias… So she's like my sister," Garner said of her reunion with Duggins. "So [it's been great] for the two of us to get to be together every day, to train together, to go back to all of that."
For more on Garner’s intense training and a look at some impressive fight moves with her stunt double, check out their full interview on Entertainment Tonight on Monday, Aug. 20. Check here for local listings.