Without even pausing to think, Ger Duffy firmly states that he had 37,000 extras on the final season of 'Game of Thrones'.

That's about 12,000 more people than can fit inside Thomond Park, and more than half the capacity of Aviva Stadium. Working as a Crowd AD - that's assistant director - on 'Game of Thrones' is a job like no other, and Ger Duffy believes the size and scale will never be seen again. "The closest we had to it was season six when we had the Battle of the Bastards, and we had 9,000 that year," he adds.

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Ger's work on 'Game of Thrones' involved working with and corralling extras through huge sequences, from 'The Bells' to 'The Long Night' and everything in between.

"I was the crowd third AD, in charge of all the background action. Anything you see involving people that's not an actor was choreographed by myself and my team," Ger explains. It's tough work, and much more industrial than you'd expect.

"You work with loud hailers, we call it the 'Voice of God', and I'd have about 20 or 30 trainee ADs who'll key up extras to run at a specific time. It's about getting people through hair and make-up for about eight hours, then another eight hours on camera, and then three hours wrap-out. So, yeah, it's a long day."

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A lot of it comes down to budget - and this was something that 'Game of Thrones' had in spades. "Instead of making 20 or 40 extras look like thousands, we actually got thousands," Ger laughs. "We still had to do a lot of tiling and VFX work to make it look like hundreds of thousands, but it's a lot, a lot of people."

More than that, the different units had to work together to recreate the different scenery - all in Belfast so they could shoot it in the one place. The likes of Iceland, Malta and so on were replicated in Belfast, complete with the same architecture and scale. "I don't know if it'll ever be done to that scale again, not at this scale," Ger admits.

By far, the most intricate scene in terms of Ger's work was one people may not expect - the infamous banquet scene in 'A Knight of the Seven Kingdoms', which featured the infamous Starbucks cup.

As Ger explains, the scene involved so many people and with veteran director David Nutter behind the camera, the various angles and takes from it meant that it was moving across the script from different stages. "You'd do one scene where it'd be, say, 8PM at night, and then David (Nutter, the director) might move on to another section that's at, y'know, 2AM or something - and you've got to remember where everyone is, change them out if needed, and then make them seem like they're more drunk or less drunk, depending on the time of night at the banquet."

For all of the brouhaha surrounding the goof, there's so much more to 'Game of Thrones' and the work behind the scenes. "With extras, there's different levels of acting. Some have acting experience, some don't, so we have to give them backstories, work with them to make them viable and believable in that scene - and with this show, we have pre-visualisation so we can see where the camera's going to be, what it's going to look like, so they can act without what they're going to see."

Yet, for all of this intricate stagecraft and moving parts, goofs happen - and so it comes to the Starbucks cup.

"First of all," he laughs, "it's not a Starbucks cup! There's no Starbucks anywhere near the place!"

According to Ger, the issue of leaving something like a cup in shot happens by complete accident - what with producers and the like coming in and out of shots, or even just actors themselves not remembering to take it out of shot. "They're all blaming each other at this stage!"

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"I don't know how it made through post," he adds, with a chuckle. After all, it went through grading, editing, post-production, and all of it - and yet still it remained.

"There's a first AD who worked on the sixth episode and we were discussing it the other day. Each scene is intricately gone through so many filters, criticised within an inch of its life - I refuse to believe that no-one saw that cup! Either it was left on purpose for some reason, who knows, I think they decided to leave it in for the laugh. Had to have been!"