Every actor knows that taking on a new role often doesn't just require mental preparation, but a total-body physical transformation, too—and in some cases, those transformations are extreme.

Tom Hardy bulked up like crazy to play Bane. Mark Wahlberg gained more than a few pounds for Deepwater Horizon. Christian Bale gained and lost...and gained again. Point is, there's no shortage of Hollywood heavyweights willing to put their bodies on the line to get camera-worthy.

And now, thanks to an intense fitness regimen and some help from celebrity trainer Don Saladino, we can add David Harbour to our list of extreme celebrity fitness transformations.

The Stranger Things actor had to go from flabby to fearsomely jacked to succesfully achieve the look of an occult-battling demonic hero in the new Hellboy reboot.

And if the before-and-after shots are anything to measure against, the guy has clearly been putting in the work at the gym:

For a behind-the-scenes look at Harbour's workouts with trainer Don Saladino, check out the video below:

Hellboy, directed by Neil Marshall and starring Harbour, Milla Jovovich, and Ian McShane, will hit theaters on Jan. 11, 2019.