There were some holes that needed to be filled, so in order to complete them, Carey sort of started writing. Through this process, she was able to create a backstory, which was really helpful.
There are a lot of facets to her. Although I believe many viewers already anticipate her to be the show's antagonist, she said that by casting her when she was younger, there was much more to explore.
However, it appears that such remarks incensed Rhaenyra superfans, who filled Carey's Twitter page with critical remarks. Carey was charged with not understanding the original material well enough, which was the spin-off book "Fire and Blood" by George R.R. Martin.