Recommendations

same actress, and both stories about modern women in the work field!

Both drama's focus on a womens ambitions in the work world, but also are super good, heart felt slice of life dramas that teach important lessons

Similarities:
#Alive is a movie centered around a zombie outbreak around apartment building and having to survive within the apartment building. Happiness is also about zombie outbreak and will also be centered around an apartment complex.

Both are dramas involving a band, as well as a single female living with male band members. Both dramas also involve twins switching.

Both are Korean dramas involving a band, as well as a single female living with male band members. Both dramas also involve twins switching.

Both dramas involve a band and a single female living with male band members. Both dramas also involve twins switching places.

Both are strong women, both the woman suffers because a family lost, both the woman is smart and work very well, both the woman built a great industry with her own efforts.

Both are set in a fantasy world where the zombie apocalypse is eminent. Both dramas starts by showing how the outbreak supposedly started and how the people around them adapt to the situation. The origin of the pathogen is different but it still has the same effect. Not your typical zombie shows.

Like the Japanese Ballad of Narayama, Kim Ki-young’s Goryeojang (???) envisages a world in which the old are expected to sacrifice themselves for the young, but unlike either Keisuke Kinoshita or the later Shohei Imamura, Kim struggles to find nobility in adherence to such a cruel and inhuman tradition.

Both horror films go around a possession case set in a village/rural area, either from Thailand or South Korea. As The Medium's producer, Na Hong Jin's work constitutes a big force of influence, making it obvious its resemblance with The Wailing.

Both dramas follow lives of people who, after their busy work, head to a bar for a bottle (or two!) of soju. Both dramas focus on drinkig culture, food, everyday life issues and crazy epic friendship dynamics (for WLDN it's 3 main female leads, for DS it is Noryangjin male trio)

Let me be your knight is about idols struggling from psychological and metal health...imitation is also a story about the struggle idols have throught their career..they also shown people struggling while debuting and after debut and even after success and also even if they don't get opportunities to show case to the world .. and one thing is comman in both its an idol theme drama and main lead is lee jun young (U-KISS)

Both are dealing with serial killers though I saw the devil has a revenge element in it.
Both has Cat and Mouse game between them.
(I saw the devil is much more gory and Insepctor Koo doesnt contain it)
(Inspector koo's serial kerial K kills the targets in such a way that it appears as an accident)

If you like "I saw the devil" try Inspector Koo, you might like it.

Both are dealing with serial killers though Signal has a Sci-Fic element in it.
Both have action, suspense, thriller elements.
If you like Signal, try Inspector Koo and there is good chance you would like it.

Both these contemporary dramas are sequels of period dramas.
The common point is that the main leads features in the dramas are reincarnations of the prequels.

The difference is that Forever and Ever is a full fleshed drama while Good Bye My Princess: Special is made up of snippets.

Both dramas have an outspoken female lead and have a mixture of comedy, mystery, action and romance