"On Yvonne's maybe second day [filming], she had to climb to the top of this 200 or 300-foot tall power plant in Atlanta and essentially do a catwalk," Pratt, 41, tells PEOPLE in this week's issue. "You're on a wire suspended on a rig, but if you fall, you're actually going to mess yourself up because you're going to be careening off a beam before they catch you. And she did it seamlessly!
"I had been practicing this thing all day," he continues. "I had absolute vertigo. I was freaking out. You could tell that for a brief moment she was nervous and she just zipped it all up, got super tough and nailed it right away. It was pretty impressive."
"Zipped it up including the poop in my pants," jokes Strahovski, 38.
In the futuristic thriller, which involves a ferocious alien race trying to wipe out humanity 30 years in the future, Pratt, who also serves as the film's executive producer, plays a military veteran/high school science teacher and Strahovski a scientist/military leader trying to save the planet.
"We did do a lot [of stunts]," says The Handmaid's Tale star. "I remember being very impressed when Chris did a one-handed slide down a rope coming out of the helicopter, while shooting your semi-automatic. That was super badass."
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Pratt refers to Strahovski's character as "the generalissimo."
"She's the commander. And also there's a great bit of tenderness and vulnerability there, which oftentimes is not written into those types of roles," he says. "We needed the right actor to play that role, right actress, but everyone, she just killed it.
The Jurassic World actor says he enjoyed helping shape certain elements in the script and being on the other side of the casting process, as the star and executive producer.
"It was really fun to be on the opposite end of the casting call because my whole career, for the better part of a couple of decades, I was at the mercy of casting decisions," says Pratt. "It was really illuminating to realize that just because you didn't get the job, it doesn't mean you're not a great actor. There are terrific actors that we saw, amazing actors that you see and for whatever reason, they don't get the part because maybe it's just not a good fit or it just didn't work out. It probably would have saved me some anxiety because every time I did not get hired, I'm like, 'Oh, I must suck at acting.' It's not the case."
Pratt says being an executive producer was really rewarding.
"It feels a little more like my baby going out into the world with this film," he adds. "I'll be paying extra attention to people's reactions because I've put a lot of sweating and heart into this thing."
The Tomorrow War is available to stream on Prime Video July 2.