Game of Thrones: Behind-the-scenes documentary's top secrets
Secure phonelines and codenames are usually the preserve of governments and secret services. But when it came to Game of Thrones spoilers, the producers of the fantasy mega-hit weren't taking any chances, as the woman who made its official behind-the-scenes documentary found out.
Filmmaker Jeanie Finlay, whose work has appeared on BBC Four, usually tells "intimate stories" in her films.
So she might not be the obvious candidate to rub shoulders with 2,000 crew members and a cast of White Walkers on the set of the world's biggest TV show.
But, to her surprise, that's exactly what happened.
"I got this mysterious e-mail from this guy who used to work at the Irish Film Board saying 'HBO are going to call'," she said.
By her own admission, Ms Finlay "wasn't the biggest Game of Thrones person", although she "knew 'winter was coming' and who Jon Snow was".
Nevertheless, she took the call and, about a month later, found herself meeting the show's producers in Los Angeles pitching her idea.
"They gave me enormous trust," she said.
"Three weeks later they said 'welcome to the family'."
A fortnight after that, she was "on a recce in a very windy quarry in Northern Ireland with the producers".
Embedded on the set, she would go on to chronicle the creation of the show's final season and what it takes to bring the world of Westeros to life.
And while millions around the world would watch the finished product, Ms Finlay couldn't tell a soul about her top-secret assignment.
Speaking on Mark Kermode's weekly podcast, Ms Finlay said she kept her involvement hidden from friends and family for 14 months.
But there was little danger of her revealing dreaded spoilers, as she had to sign a non-disclosure agreement (NDA).
"It was such a secret - even the existence of the film was a secret," she said.
'Dragons and ice lakes'
In scenes which could have come from other HBO dramas, the US network even installed a dedicated phone line at Ms Finlay's office in Nottingham's Broadway Cinema.
"Three guys from HBO came over from New York, they got the road dug up, they put in a special secure line, we used code names and cut on encrypted material," she said.
"We had to use code names for all of the actors because of spoilers.
"It is actually surreal to be able to talk about it.
"I'm so relieved the finale has gone out."
For Ms Finlay, whose films, such as Orion: The Man Who Would Be King, tend to be fly-on-the-wall portraits of single subjects, the scale of Game of Thrones' production was a shock.
"I'm a documentary maker so this was something else," she said.
"It was just huge. It was four months before the cast turned up."
"They had dragons and ice lakes and a castle - a massive castle which can be shot from any angle.
"There are costumes which had acid-etched decoration, the level of detail is extraordinary."
In her documentary, the filmmaker tells the stories of eight people, including Vladimír Furdík, who plays the Night King in the show.
"He's the biggest, baddest, villain in Game of Thrones, but he's the most adorable Czechoslovakian stuntman," she said.
Despite not being a fan going into the project, she fell in love with the Game of Thrones world, and even got her 15-year-old daughter a part in the final episode as a wildling.
"Just the scale of it is so amazing which makes it terrifying as a job to do," she said.
"The producer Chris Newman said, 'You must never be intimidated by the feast, you can only eat the meal in front of you', so that's what we did, day by day."
The Last Watch, by Jeanie Finlay, can be seen on Sky Atlantic.
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