Gu Family Book because of Lee Seung Gi. :) (But its one of the best dramas Ive ever seen)
Yes, almost every dramas that I've watched :DD I usually start drama when there is a one or more actors, that I like.
I think it depends . I Usually Watch dramas beacause of the synopsis but when I know that one of my favorite actors are going to play in a drama I will Watch it ( for exemple " Arang and the magistrate" & "Two weeks" for Lee Jun Ki , " Innocent man" for Moon Chae Won & Jong Ki , " Shark" for Nam Gil & Ye Jin , " cheongdamdong Alice" for Park Shi Hoo , "Kazoku game " for Sakurai Sho etc .) For me, the most important is the plot ^^
Of course... I watched King2Hearts only because of Ha Ji Won and I wasn't disappointed. Good drama, but I have yet to finish it.
When I first started watching kdramas I had no idea who was who and I thought it would stay that way. When I read about people watching shows because of their favorite actors I thought it was funny. Then I started recognizing a few names here and there, later I thought I'd check out these old shows of Geun Seuk or Min Ho, you know, just for fun. Now, years later, I've watched all kinds of nonsensical dramas just because of actors!!! My brain tells me it's silly but still, they drag me into it! Its happening right now, Good Doctor is just plain bad and yet I have a feeling this time, Joo Won will get the better of me and take me for the whole ride. My kdrama shame list has Love Rain, Nice Guy, Baker King, Gu Family Book but the real champion is LMH with 3 bad shows in his belt, BOF, Personal Taste and Faith. I shiver thinking of Heirs.
It would be a lie if I said that I didn't watch some dramas because of the actors but there are other factors that are just as crucial. Maybe I should say that having favorite actors in a certain drama is sufficient but not necessary condition for me to watch a drama. I forgot to add that I'm currently watching Boys Before Flowers just because of Kim Bum. XD
I only watched movie because T.O.P and Seungri :D
all the time. i search my favorite actor's drama lists and watch them , sometimes i watch drama coz someone recommended it but that rarely happens
I usually watch a drama just because of the actor but i end up loving it =)) it so happens that my fave actors/actresses know what drama to choose.
the musical because of ( koo hye sun & choi daniel )
The first few dramas I watched, didn't know who was who, but fell in love with Lee Min Ho when I watched Faith (my 3rd drama) then watched everything of his... found Kim Bum and Kim Hyun Joon in BOF, watched all theirs, found a few other good actors, watched all theirs, etc etc. I've watched quite a few based on synopsis, like Missing You, and found even more actors/actresses I had to see everything by. I have told others, and it's true, I'm a Certified Stalker :P I see all the dramas, the movies (if I can find them), the CFs on Youtube, interviews, variety show stints, etc.... I'm so pathetic LOL
Yes!!! Beethoven's Virus because of Jang Geun Suk... that was a terrible decision...
Boys over flowers i discovered Lee Min Ho then because of him I watched personal taste and I loved it.
Yamamoto Yusuke, Miura Haruma, Lee Jun ki and other japanese/korean actors, have i watched some with. Even though i have watched a handfull of dramas with them i don't watch dramas without a decent plot.
Yes. If I see an actor/actress I like, I'll try to watch something more with him/her - but usually I don't watch a drama/movie only because of the actor if I don't like the synposis. But I will watch everything I can with Iura Arata!! Usually that means something good to watch... but May Your Love Bloom A Hundred Years was so not worth it (he was in it for like 2 minutes). Last night I also watched Kimi ni todoke - that, on the other hand, was a very good movie! :)

Lately I've also begun watching things with Fujiki Naohito. Currently watching Control, I quite like it. I tried watching Yako no kaidan but I dropped it.

Edit: Well, these titles mentioned are all Japanese, not Korean... but I guess I would react the same way if I found a Korean actor/actress I liked. Actually, I wanted to watch more movies/dramas with the woman who showed up in the last minute of Sunny, but I guess she was just an extra..... just my luck!