"The first time feeling that drop and the noise of the barrels being pulled back is terrifying when you're standing above it," he says.
"It feels so real and that adds to the ability to act terrified during the scene."
Showrunner David Benioff says the other reason the scene was so emotionally charged was because it relates back to Ned Stark.
"The man who passes the sentence swings the sword, it's the northern ethos," says Benioff.
"Jon really doesn't want to do this, but the justice of if it is clear - these men assassinated the Lord Commander, they need to be executed."
In the featurette, they also go behind the scenes of Dany's fiery ending in the last episode. You can watch the full video here: