The opening scene of “House of the Dragon” shows an ailing King Jaehaerys deciding on his successor — Viserys or Rhaenys — but in the book, Rhaenys never came so close. In Martin’s writing, the choice is between Viserys and Rhaenys’ son, Laenor; the show likely made the change to simplify the story and highlight the difficulty of women gaining power.