Margaret John, who played Old Nan, died a couple months before the show began airing. "Lord Snow," where she first appears, was dedicated to her memory.
Wilko Johnson, who plays Ilyn Payne, has terminal pancreatic cancernote He's since undergone a radical surgery that may have saved his life, but it is unlikely that he will recover in time to reprise his role all the same. . As a result, his character hasn't been seen since Season 2. In Season 4, his role from the books in training Jamie to fight with his off-hand has been taken by Bronn.
Absentee Actor: With such a huge Ensemble Cast, this is inevitable. Only two actors appeared in every episode that aired before their characters were killed off: Mark Addy (Robert Baratheon) and Sean Bean (Ned Stark).
Ascended Meme: Fans often mocked the less than original "stolen dragons plot" in Daenerys's Season 2 storyline with cries of, "Where are my dragons!?". Time Warner Cable turned it into an ad.
Sophie Turner went from blonde to redhead for Sansa; Joe Dempsie and Alfie Allen went from blond to black, while Peter Dinklage and Jack Gleeson went from brown/dark blond to light(er) blond to better portray members of the extremely light-haired Lannister family. A lot of the other actors, like Emilia Clarke and Lena Headey, wear wigs. Meanwhile, Maisie Williams cut her hair to convincingly portray Arya trying to pass as a boy in Season 2.
Daniel Portman is a sportsman in real life, but let himself go in order to be more believable as Podrick Payne.
Gwendoline Christie apparently really got into the role, putting on lots of muscle with the aid of protein shakes and going up a clothes size, according to Sophie Turner.
JorahMormont is commonly dubbed "Ser Friendzone" or "King of the Friendzone".
Tyrion "The Kingslapper", due to his habit of bitchslapping Joffrey.
Theon's mysterious torturer has been nicknamed Barry and "Little Bastard". Also goes by "Evil McFuckingcrazy" in some circles.
Podrick is called "Tri-Pod" or "Prodick" after managing to pleasure three whores so well they let him keep his money.
In-universe, "Lord Twatbeard" is apparently Bronn's nickname for Littlefinger.
Daario Naharis is nicknamed Fabio (also Faabio and Desert Fabio) after the Mr. Fanservice model that appeared on hundreds of romantic novel covers in the 80s and 90s. He's also been referred to as "Zevran" by gamers, due to his strong resemblance and similar personality to the Dragon Age character.
The shock of The Rains of Castamere resulted in a massive outpouring of fan anger in social media. It was apparent that many of the casual fans couldn't spell the characters' names, so these misspellings were often jokingly adapted as Fan Nicknames by the veteran fandom. "Caliesie", "Geoffrey Lanister" and "Walter DeFray" come to mind.
Fans have taken to calling Hodor "Brodor" whenever Bran wargs into him.
Now that Sansa has finally learned how to play the game and sports much darker look, many fans refer to her as Darth Sansa.
Robert Baratheon is sometimes referred to as "Bobby B."
Follow the Leader: The success of the show has inspired imitators, notably Vikings and The White Queen. The latter is particularly notable, as it's a retelling of the Wars of the Roses, which inspired much of the initial plot of Game of Thrones.
Lying Creator: In one of the featurettes aired prior to the start of Season 1 to introduce the world of Westeros to viewers, George R. R. Martin said that "Ned Stark is the main character whom the books revolves around." In reality, he's only the main character in the first book. At the end of it, he dies, and the rest of the books have no real main character.
Practically every cast member who's done an interview for the show will go out of his or her way to say how lovely a person Jack Gleeson is. You could make a Drinking Game out of it. When he's recognized or asked for autographs, he's even been known to joke "I'm a nice guy, I swear!" Sophie Turner (Sansa) relates how Gleeson asks her if she is all right the moment the cameras stop rolling after some abusive scene.
Charles Dance finds Lord Tywin's appalling treatment of his son awful. He remarks that it's hard for him to mistreat a nice man like Peter Dinklage.
In the show, Cersei Lannister loathes her brother, Tyrion. In real life, Lena Headey and Peter Dinklage are very close friends and live together during filming.
Michael McElhatton (Roose Bolton) is also a very decent guy who plays a sadist and a traitor. He says that he found shooting parts of the Rains of Castamere very emotional. In his native Ireland, he was known more for comedy before taking on the role.
Thor Bj÷rnsson, the third and arguably most terrifying man to play monstrous Hero Killer Gregor Clegane, is nothing if not hilariously jovial, as demonstrated by the collection of goofy Instagram photos he has of himself and his costars. Including Pedro Pascal, whose character he murders in possibly the most gruesome scene of the series.
Conan Stevens only had one appearance as Gregor Clegane in the first season before leaving the series to appear as Azog the Pale Orc in The Hobbit. He's replaced by former basketball player Ian Whyte in the second season. Apart from their height, the two actors look nothing alike, not to mention that Stevens is quite husky, while Whyte is much thinner. Quite a few people seem to have missed the fact that he was supposed to be the Mountain. The Mountain gets recast again in Season 4, where he is played by Icelandic bodybuilder, Haf■ˇr ´┐ŻThor´┐Ż Bj÷rnsson.
Beric Dondarrion is played by David Scott in first-season episode A Golden Crown and by Richard Dormer when he reappears in third-season episode And Now His Watch Has Ended. This was somewhat less jarring, as the previous actor had such a minor appearance and was so inconspicuous in looks that fewer people remembered him; furthermore, the character has been through a severe Mutilation Conga in the interim.
Tommen Baratheon was played by Callum Wharry in Seasons 1-3, but beginning Season 4, he will be played by Dean-Charles Chapman, who previously played Martyn Lannister in Season 3.
Michiel Huisman replaces Ed Skrein as Daario Naharis in Season 4. The two look almost nothing alike.
Keeping with Tommen's recast, the role of his older sister Myrcella will go from Aimee Richardson to Nell Free in the fifth season. Bizarrely, Free is two years younger than Dean-Charles Chapman, putting the recast in similar territory to that of Gregor Clegane.
The Other Marty: Even before it began airing, it happened for a lot of characters between the original pilot and the series proper (most of the pilot was then reshot with the new cast).
Daenerys went from Tamzin Merchant to Emilia Clarke
Catelyn went from Jennifer Ehle to Michelle Fairley
Magister Illyrio went from Ian McNeice to Roger Allam
The show's 2013 April Fool's joke was a fake announcement that Peter Dinklage would be replaced by Warwick Davis to make Tyrion a more "comedic" character. The showrunners also claimed that they would digitally transfer Davis' face over Dinklage's in reruns and subsequent DVD releases.
Overtook the Manga: As of the fourth season in 2014, Game of Thrones is coming dangerously close to overtaking the novels. While there are still two hefty novels left before the show runs out of material, both of those books occur at the same time and several characters have already started to dip into the storylines from those novels. George R. R. Martin has given the show's writers a detailed explanation of events in the final two novels, as the writers and producers have made it clear they will not and cannot stop and wait for Martin to catch up. The Season 4 episode Oathkeeper is the first to feature a major revelation before the novels: the final scene reveals the Others/White Walkers have a society and hierarchy, that they create new Others/White Walkers by transforming human babies, and that they are led by the Night's King, an evil figure from Westeros mythology who was not previously confirmed to actually exist. Martin got himself into some hot water at the same time by giving a few interviews where he appeared convinced that he could finish the series in time for the show to adapt it, which he later clarified was "pie-in-the-sky" wishful thinking.
Promoted Fanbaby: Gilly's son is portrayed by 10 months old Arya Hasson. Guess where her parents got the name from.
Real-Life Relative: Daniel Portman (Podrick Payne) is the son of Ron Donachie (Rodrik Cassel).
Romance on the Set: Apparently, Kit Harington (Jon Snow) went on a date with Rose Leslie (Ygritte).
Throw It In: The sword flip where Syrio Forel balances a weapon on the back of his hand in his first scene with Arya was improvised. "Just so." Even the actor was surprised when he managed to do it, because the wooden sword was considerably heavier than a normal one.
In a commentary for Season 1, the producers talk about the difficulty of adapting certain things from the books, and specifically tell us not to get our hopes up that white ravens will be included. In episode one of Season 2 (filmed before the commentaries were made), we do get a white raven.
Lena Headey (Cersei Lannister) is fond of obliquely spoiling future plot developments on Instagram. She also messed with people's expectations when she posted a heart-shape made of stones, which teased that Lady Stoneheart would appear in Season 4. She didn't.