Ian McElhinney, who plays Barristan Selmy, was surprised to learn of his character's death, as he had read the novels where Barristan was still alive at that point, and becoming an increasingly prominent character with his own point-of-view chapters. McElhinney suspected something when his shooting schedule was shorter than usual; his death scene was originally scripted to occur in Unbowed, Unbent, Unbroken (2015), but got pushed ahead during production. In an interview after the episode aired, McElhinney remarked, "He had to be seen fighting. He's been talked about as the greatest knight that ever was, so he's got to fight. So it's great that he does." In the last book that was released at the time, Selmy was gearing up to fight a battle, and it was speculated that he might die in the unreleased next one.
There was a lot of discussion as to whether Barristan Selmy should be kept alive, as he was in the books. When he found out about Selmy's planned death, actor Ian McElhinney himself even argued with the writers to keep Selmy on the show longer. However, it was decided that his death needed to happen so that Daenerys would be increasingly isolated as the season went on, and also to make room for Tyrion to become Daenerys' trusted advisor later on.
Ser Barristan kills 13 Sons of the Harpy before succumbing to his injuries (14 if you count the one he uses as a human shield against a sword attack); Grey Worm kills at least 16.
As no major characters were located there, originally, there were no plans to go to Dorne in season five, until Bryan Cogman came up with the idea of sending two established characters there and pairing Jaime up with Bronn.
Keisha Castle-Hughes is the first Oscar-nominated actor to star in the series. Max von Sydow, who joined in the sixth season, would be the second, and Jim Broadbent (seventh season) the third (as well as being the only winner of the award). Jonathan Pryce, Richard E. Grant and Ciarán Hinds would become Oscar nominees after their stint on the show.