Park Eun-bin received the script for The King's Affection at a time when she was thinking about acting in a historical drama again, as the last one she filmed was Secret Door in 2014. While aware that there would be other opportunities in the future to fulfill her wish, she decided to take the role because the premise of the series was very attractive and she doubted she would have another chance to play a king. At the press conference before broadcast, Park commented that despite her taking on the role of Lee Hwi with confidence, she had had a hard time making it convincing because it was the first time a woman played a king. Instead of trying to simply disguise herself as a man, she attempted to focus on showing why Hwi had to pretend to be a male and on the moments in which she faced doubts and dangers, as her real side slips out, declaring that she wanted Hwi to be seen as a person regardless of gender. She also took riding lessons and paid attention to adjusting the tone of her voice according to the people around Hwi.
