If you're thinking "Pfffffft, sure I've already seen Jon Snow's bottom on that one episode of Game of Thrones" - you haven't. Sorry to break this to you, but he used a bottom double for this shot.
He wasn't being precious; he would've done the scene himself, only he reportedly had a broken ankle at the time, so launching himself into a rock pool with Ygritte wasn't exactly a goer (although fair play to the stunt double in question - there is a definite likeness given the below photographic evidence of Harington's actual bottom)
When you enter the world of theater, it's not quite as easy to substitute one bottom for another one, given the live element. Therefore, West End theater goers have been captivated by Kit Harington's posterior during his performance in Doctor Faust.
While a lot of audience members simply chose to pen their appreciation via twitter...
seeing kit harington's bum in person is the highlight of my week
— olivia (@oliviasjacksons) April 21, 2016
kit harington's bum
That is all
— em (@kitharingtns) April 18, 2016
... one intrepid soul got the shot a lot of people have been looking for. In sort, Kit has a very lovely bottom.
— Best Bums (@best_bums) April 17, 2016