Game of Thrones season 7 episode 6: 'The Frozen Lake' YouTube 'making of' video shows how they built a fake lake

HBO's Game of Thrones 'Inside the Episode' videos have given a fascinating insight into the practical and visual effects challenges that face the production team when trying to stage such fantastical and big scale scene as the ones seen in season 7.

Episode 6, 'Beyond the Wall', was particularly epic, seeing Jon's splinter cell surrounded by wights on a frozen lake.

Director Alan Taylor discussed how tricky it can be to tell a story amid stuntman and green screen chaos.

"Our story is seven guys on a rock being attacked by wights, and trying to tell the clear story that your task is to defend a perimeter and, as you lose, your perimeter's going to fall back and fall back - [so it's about] how do you hold order against pure chaos?

"It's like herding kittens, you can't see anything except people whacking each other; it takes some effort to talk to the cast and say 'No, the story is 'here's the line and you have to show that it's not being broken or that it is.''

The video also revealed that a wight bear is something the showrunners have been trying to include for a while.

"What we had to go through to get the zombie polar bear was writing the zombie polar bear into every season of the show for about four seasons," D.B. Weiss explained, "Only to have Bernie and the visual effects guys, in the nicest possible way, say 'Fuck you, we cannot afford a zombie polar bear.'

The Hound actor Rory McCann also discussed how weird it was pretending a bear was there on set when in fact it was a guy named Toby in a green morph suit and the practical effects team told how they used three and a half thousand bags of fake snow in making the frozen lake (actually a cemented over quarry) look real.

"I thought it was a real lake," Tormund actor Kristofer Hivju confessed.

Game of Thrones season 7 concludes on HBO, Sky Atlantic and through NOWTV on Sunday night.