After reading Hyeyoon's interview, it seems she was truly sincere about her role as Suin in Salmokji.

I saw an interview that came out today, and Hyeyoon said she tore off all her bright energy for her acting this time.
He loves horror movies so much that he was even excited enough to wait for a ghost on set. Isn't that just adorable?
It touched my heart the whole time I was reading because you honestly shared your thoughts on turning thirty and your concerns as an actor.
Seeing Hyeyun constantly wanting to learn makes me look forward even more to what she will show in the future.