David Harbour Posts His Body Transformation for 'Stranger Things' : 'A Difficult and Exciting Ride'

David Harbour is revealing details of his Stranger Things body transformation.

On Instagram on Thursday, the 47-year-old actor opened up about what went into his character Hopper's slimmed-down look for the Netflix sci-fi show's fourth season.

According to Harbour, he and his trainer David Higgins worked "for 8 months to make the transformation, and then another year to keep it through the pandemic."

"All told it was a difficult and exciting ride," he admitted, "changing diet and exercise plans (or lack thereof)."

The actor's post came with a series of photos from the process, including a before-and-after split of him on the show. "Second pic is me and David the week we began, my resistances and fury flaring," he wrote, of a selfie in which he was flipping off the camera.

A third shirtless photo showed Harbour taking a shirtless photo in front of the mirror, his torso covered in black x's he said were for cgi scarring.

Overall, Harbour lost more than 75 lbs, "265-270 in season 3 and all the way down to 190 when we shot."

Also in the caption, the Hellboy actor said he regained the weight for his upcoming movie Violent Night where he plays Jolly Old St. Nick. "I can't wait for you to see this holiday season," he said.

"I am struggling to fight back down towards a good weight for wherever Hopper ends up in season 5," Harbour admitted. "All this up and down is not good for the body, and I'll have to give it up soon, but it is such a fun part of the job to live in a different version of your skin for a while."

Harbour will be going live on Friday with his trainer to answer fans questions and share some advice.

In an interview with British GQ earlier this month, Harbour gave more details about his weight change.

The actor explained that in order to properly portray Hopper — who begins the new season held captive by Russians at a labor camp — he lost the weight by doing intermittent fasting and Pilates.

"I don't think I'll ever do that again," he told the outlet. "I have this Santa Claus movie [Violent Night] coming out for Universal in December and I gained [it all back]. But now, yeah, never again. The prosthetics are too good."

Before beginning his journey, the actor had put on weight for his role in Marvel's Black Widow, he told The New York Times.

"I was like, 'Let's use the weight.' So I started eating even more. I got up to 280 lbs., and I loved it," Harbour told the outlet.

Right around then, he met singer and now wife Lily Allen.

"It's a true testament to my undeniable charisma when I say that my wife met me at 280 lbs. with this beard and this hair," he said, dryly. "We went on a date at the Wolseley [restaurant] in London, and she really fell for me at my worst, physically and hair-wise."

And after dropping those 60 lbs., Allen had "mixed feelings" about her husband's new look.

"She honestly has some mixed feelings about it. Which is a good place to be in a relationship," he said. "It's really good to start the relationship from that part, as opposed to being the young, handsome buck and watching yourself degenerate over the years."

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Allen, though, is big on complimenting her husband. In 2019, after Harbour was named one of PEOPLE's Men of the Year in the annual Sexiest Man Alive issue, he shared one of the best things Allen has said to him.

"Lily Allen once told me that I was so gorgeous it was an outrage," he told PEOPLE.