Behind the fantasy that is Game of Thrones, there’s the reality of all the hard work that goes into it - especially when it comes to creating the White Walkers.

As GoT heads into its sixth season, HBO has released a featurette of everything involved in making the White Walkers and other characters as scary as they are. In the video, the show’s prosthetics supervisor Barrie Gower, whose credits also include the Harry Potter series, explained that to make just one character, “We usually say an average kind of build time is about four weeks.”

Gower added that on a daily basis, it can be five or six hours from the moment the actor gets in the make-up chair until they start filming for the day.  Then there’s up to 10 hours of actual shooting. Finally there’s the removal, which adds a couple more hours.

The new season of Game of Thrones premieres Sunday, April 24, but you already knew that.

Check out the video.

David Eckstein is a writer living in Los Angeles.