Ther actor plays Jim Hopper in the hit Netflix series, and his storyline in the latest season saw his character imprisoned in a Soviet labour camp.
For the scenes, he ”lost 80 pounds” (five stone) in eight months by fasting intermittently and doing Pilates.
“I was about 270 [then], and when we shot season four, I was around 190,” Harbour told GQ.
However, he added: “I don’t think I’ll ever do that again”.
Harbour said that he “gained it all back” to play Santa Claus for a forthcoming film titled Violent Night, stating: “But now, yeah, never again. The prosthetics are too good.”
Elsewhere, the actor suggested that spin-off projects will be announced closer to the final season’s release.
“I think as soon as the show ends, or maybe six months before it ends, you’ll be hearing about whatever spin-offs they have planned,” he said. Harbour also revealed who he would want to play a younger version of Hopper.
On a new podcast interview, the Duffer brothers, who created the series, defended themselves from criticism of the series by two of its stars, including Millie Bobby Brown. The pair also shared new details about what fans can expect from the spin-off series, shutting down two prominent rumours.
They also revealed the biggest way the fifth and final season will differ from the fourth.
Meanwhile, it was revealed that Joseph Quinn, who played new character Eddie Munson, improvised one of the season’s best lines.
Stranger Things is available to stream on Netflix now. The final season is expected to be released in 2024 – find out everything we know about it here. and The Independent’s verdict on season four, volume two here.