The main serial case in both dramas have a lot of similarities, in which it may be based on the same murder case that sparked many different dramas and movies based on it. In both dramas there seem to be some hidden conspiracy which lead to people attempting to cover up the murder cases and led them remained unsolved.

Well, both dramas were from the same screenwriter thus you can expect similar style of wit and comedy in the plot. And both drama had a group of people acting and behaving as a part of some mafia-like cartel (with Vincenzo actually was one himself).

And while Vincenzo only just started, I can already find a lot of similarities between the Gootham gang and the Geumga Plaza people. Heck, as much as Hae-il and Vincenzo were quite different from each other, the two of them did share some similar traits in personalities. Not to mention, Gyeongseon and Cha-young were also very similar in personalities and weirdness though profession-wise they're the opposites.

While the two dramas have different plot, the two did share some similarities in the setting as well as overall plot. Both Kangto and Hoon had a tragic past in which their lives took a drastic turn during their childhood, which lead to them taking a path that wasn't quite righteous. And it's due to the sacrifices of their loved ones (Gangto's older brother and Hoon's father) that they ended up trying to change their lives into being a lot more true to themselves. Both Kangto and Hoon inevitably being dragged into a political strife that more or less shaped the lives they ended up in and both also were involved in a tragic love relationship with the girl they loved. Not to mention the fact that in both dramas Jin Seyeon played as both of the protagonists' love interest.

Both are medical themed dramas where the main protagonists attempt to change the stagnant medical world using their own method as doctors of their own rights while not being chained by any rules and refusing to back down to other's pressures. While their main fields of experties are mostly on the medical sides, both Daimon Michiko and Shiratori Keisuke can be quite a detective in unearthing the corruption in the environment they work in and be quite analytical in trying to resolve the obstacles they encountered.

Both drama had similar main character types that carried certain roles; in The Guest, Hwapyung was the one with the psychic abilities, Yoon was the one mainly tasked with the exorcism and Kilyoung was the detective that allowed them to investigate crimes commited by the possessed people. In Cheoyong, he was the one taking up the role of a psychic, exorcist and detective with an extra help from a lost ghost and a team of detectives.

Both dramas had the main characters hunting the main evil spirit that was the cause of every incident in the drama. Also, in Cheo Yong season 2, the actor Kim Dongwook who played as Yun Hwapyung in the Guest played a character who ended up being possessed by evil spirits and kill people.

Both dramas had one or more character having the abilities of precognitive dreams and for at least one or more of the characters, the dreams manifested into nightmares. Both dramas have the characters who had the precognitive dreams trying to change the outcomes of the nightmares that they had seen in their dreams.

Both feature a supernatural character who were tasked to tailor make clothes for someone in order to console people. In Black Knight, it was Sharon who needed to make clothes in order to redeem herself from her past sins. In Handmade Love, Wobeun needed to make clothes for humans so that he could finally be allowed to return to Heaven.

It's basically the same plot except that The Guest is much better written than Possessed. And it's what people who initially started watching the drama would've hoped to see from the drama before the writer decided to give up on the plot of Possessed.

The Guest is basically the drama that the writer of Possessed would have wished they could replicate in terms of storytelling but failed miserably. In terms of making the antagonist, in this case the main evil spirit/demon, The Guest did it better without making the demon be too overpowered or overly exaggerated. The conclusion of The Guest also was much more satisfying and less depressing than Possessed.

Both dramas feature a psychopath who tried to pursue a life that others may deem to not suit them due to the fact that they had an inability to feel any emotions or emphatize with other people. Both dramas had the main character ending up misdiagnosed while still young and growing up having traits of psychopathy inside them. Both dramas had the presumed psychopath character trying their best to break away from the stigma people had of psychopaths in general as they try to lead a normal life.

Both feature male nine-tailed foxes who ended up falling in love with humans and wanting to become humans. Both also featured nine-tailed foxes that became evil spirits due to an unfortunate circumstances in the past.

While the two shows dealt with different kinds of supernatural creatures aka gumiho and grim reapers, there's still a point of similarity between the two which was the Korean mythology involving the afterlife/underworld.

The two drama had an almost similar plot that is a doctor time-slipping between two different time periods. But in JIN's case, it was a modern doctor who ended up time-slipping towards the past. To be fair, I would've recc Dr. Jin that was a Korean adaptation of JIN for the sake of sticking to the same Korean setting but I might as well recc the original instead.