I am European, Italian, and I watch K-dramas in the original language, with subtitles.
I certainly miss a lot about Korean culture - which nevertheless fascinates me - and in particular about ancient Korea. So my review will have many flaws. I didn't know the two actors before Captivating the king.

Indeed the actress seems a bit stiff but I think it depends on her character: even though she has become her lover, she cannot behave with too much confidence. Look at the reluctance of her gestures even after the first meeting, and how they change later, when she is now convinced of the king's sincere love and of her real intentions towards the Crown Prince. Of course the king has more freedom of maneuver. But, I repeat, I don't know of any other performances by the two actors.

The historical setting takes many liberties, but overall it draws quite clearly on the events of the second Manchu invasion of 1636 and King Hyojong of Joseon.

In short, overall I watched this series with interest - and I rewatched a few episodes back, to better grasp the different nuances, and it didn't seem so bad to me.