Good research is key. They need to dig deep into historical records, whether it's about the daily life, politics, or culture of the era they're writing about. For example, an author writing about medieval Europe should know about the feudal system, the role of the church, and the technology of that time. Another factor is the ability to create engaging characters. Readers should care about the characters' fates even though they are set in a historical context. Also, a strong narrative voice that can draw the reader into the past is important.
There are multiple elements that contribute to the success of authors of historical novels. One is authenticity. If they can accurately portray the language, customs, and values of the historical period, it makes the story more believable. For example, an author writing about the American Civil War should use the appropriate dialects and depict the social divides of that time. Creativity also plays a part. They can't just repeat historical facts but need to weave them into a unique and compelling story. And they need to have a good understanding of human nature. Because no matter what the historical era, the characters should be relatable in terms of their emotions, desires, and flaws.