Everyone knows that the hand-slap game is the literal worst, but it's probably even more inadvisable when you're playing against a gigantic dude who posts videos like this one:

A post shared by Jason Momoa (@prideofgypsies) on Oct 8, 2016 at 9:17am PDT

And yet, that's exactly what Game of Thrones showrunner David Benioff did when hanging out with Jason Momoa a few years ago, because, Benioff said, "it seemed like a really good idea at the time."

Benioff and his fellow Game of Thrones executive producer Dan Weiss told the tale at South By Southwest, where they appeared on a panel moderated by stars Sophie Turner and Maisie Williams — who got their showrunners to admit all kinds of incriminating information, including that Season 8 will only be six episodes long, and that Ed Sheeran will be guest-starring in Season 7.

"We were having a lot of fun and laughing and it was funny," Weiss recalled. "Then I remember looking at David and saying, ‘dude, look at your hands,’ and his hands had already started to swell up, because Jason is 250 pounds and so fast."

"That was the problem; I thought, ‘he's big, but I bet I'm quicker.’ But I was not," Benioff deadpanned. "So I went home — I’d been away from home for about a month, my wife is taking care of the kids with no help from me. I come limping in the door and my hand is swollen up like a catcher's mitt, and she’s like ‘what the fuck is that?’ And I said ‘oh, I was playing the slap game with Jason Momoa,’ and she was just so disgusted with me."

One trip to the emergency room later and Benioff said that the doctor pronounced, "Oh yeah, your friend squished your hand."

"The medical term for that is 'squished,'" Weiss agreed.

"So, anyway, don’t play the slap game with Jason Momoa," Benioff advised, in case Aquaman ever challenges you to a game.

Can you imagine 'Game of Thrones' without this face? Image: hbo

But when Benioff and Weiss decide to turn the tables on their actors, they generally come out on top — Turner and Williams revealed that the duo have a penchant for pranking their stars and that their sadistic streak would put George R. R. Martin to shame.

"The easiest to pull off was when we wrote a scene for Kit Harington where his face melted off," Benioff said. "There was a scene in the first season where he fights with a wight and the wight catches on fire and he burns his hand but we rewrote the script so his face actually melts off."

"It was his upper lip and all of his hair," Weiss noted.

"And Kit’s got great hair and he’s very proud of it and he’s got a great pout. And the page described exactly what his mangled face was gonna look like forever," Benioff chuckled. "He was a good sport about it, but you could tell he was really sad."

"It was good acting, he was trying so hard to not show how brutally disappointed he was," Weiss added. "We had a whole story about how HBO thought like he felt too Disney, too Harry Potter; he was just too pretty and they wanted to ugly him up a bit. He bought it for a while until David started laughing."

Among Benioff and Weiss' other pranks: Convincing Alfie Allen that Bran (Isaac Hempstead Wright) was going to kill Theon in Season 2 after the sack of Winterfell and forcing David Bradley to wear a ridiculous, Henry VIII style costume — complete with a gigantic codpiece "the size of a loaf of bread" — at a costume fitting because they convinced him Samwell was getting a new look.

Then again, the actors are pretty good at getting their revenge when the opportunity arises: Nikolaj Coster-Waldau (Jaime Lannister) tricked the showrunners into thinking he'd shaved his head right before filming was about to begin.

"He said he was getting annoyed because he wanted to get a haircut and we hadn’t approved anything," Benioff recalled. "He’s like, ‘guys, I’m gonna do it, I’m just gonna cut my hair,’ and he sent us a picture where he’d shaved his head — he looked like a skinhead, and this was a couple weeks before we were supposed to start shooting. And I was like, 'now we're gonna have to get a fucking Prince Charming wig thing.'"

Looks like he got his way eventually. Image: hbo

"We were so angry about it, HBO’s lawyers were calling his agents..." Weiss laughed.

"It turned out he'd taken a photo of himself from five years ago with his head shaved and sent it to us," Benioff sighed. "So he got us."

Good to know that everyone involved in Game of Thrones is just as cruel as the show itself.