The real 'Tomorrow War' beast is a jacked-up J.K. Simmons: 'He's got prison muscles'

• J.K. Simmons got muscled up to play Chris Pratt's screen father in "The Tomorrow War."
• Director Chris McKay remembered viral gym photographs of J.K. Simmons pumping iron and asked for a return to form.
• Simmons, 66, plays an off-the-grid former military man in "The Tomorrow War."

"The Tomorrow War" features ravenous White Spike aliens that threaten human existence. But one fearsome creature in the sci-fi action film earned true fear and respect from co-star Chris Pratt.

Jacked-up J.K. Simmons in a tank top.

The 66-year-old Oscar winner makes an impressively muscled first appearance in "Tomorrow War" (streaming now on Amazon Prime). And he only gets more formidable onscreen as the former-military father of Pratt's Dan Forester.

"It was unbelievable," says Pratt, recalling seeing Simmons on the set for the first time. "I was like, is that even possible? He's got prison muscles. I was like, 'Dude, where have you been?' " 

Working out, dude.

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Fortunately for Pratt, Simmons says he's only interested in fighting Father Time. In 2016, his training partner and friend, actor Aaron Williamson, posted pictures of a bearded Simmons crushing it at the gym that went viral. 

"That kind of blew up after it was posted on Instagram," says Simmons. "I was just trying to remain a viable romantic interest to my wife."

"Tomorrow War" director Chris McKay recalled the pictures when he called Simmons about the role of the estranged father living off the grid. Of course, the "Whiplash" actor's acting chops were McKay's prime casting motivation.

"But I was like, 'Hey, I don't know what shape you're in now. But if it's possible, I really want you to get to that jacked-up guy again and grow out your beard," says McKay.

Mission accomplished. 

The ripped Simmons was all the talk on the "Tomorrow War" set.

"He was like Coney Island strongman stuff. You expected him to be crushing cans off his biceps," McKay says.

Simmons says he received the ultimate compliment from Katherine Schwarzenegger, Pratt's wife and Arnold Schwarzenegger's daughter. Pratt relayed the direct message of: "Oh, my God, J.K. Simmons is jacked."

"That's all I need from Arnold's daughter," says Simmons.

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Simmons shamelessly fished for his own compliments from Arnold while the two worked on 2015's "Terminator Genisys." Showing the former Mr. Universe impressive pictures from his performance as the music instructor in 2014's "Whiplash," Simmons asked, "How about those biceps?"

"Arnold actually said to me, 'Your biceps were OK, but your triceps were impressive,' " says Simmons. "I'm taking that to my grave."