strong woman do bong soon for jisoo lol
The K2 only for Ji Chang Wook xD 
no the story of drama is more important for me
Punch 2003
Sung Si Kyung
Cinderella and the four knights for Jung il woo
Strong woman for ji soo
Faith , the heirs and bof for lee min ho
too many to list...its a way of life for me :D
I normally will watch dramas with Lee Min Ho, Lee Jong suk or Park Shin Hye. The rest I will pick based on ratings and other reviews.
After I watched Healer I watched few dramas because of Ji Chang Wook I enjoyed K2 just because his and the older Yoona's performance, Suspicious Partner was good but nowhere near Healer and Park Min Young surprisingly at her older dramas writing quality suddenly drops at the middle both City Hunter and Sungkyunwan Scandal started so good and then nosedived at the middle for that reason only I am skeptical about her new show (something with a queen i guess)

I watched Suspicious Partner for Ji Chang Wook, but I stayed for both him and Dong Ha
I don't typically do it. Usually, I choose the dramas based on genre and story, not cast. 

The only 3 cases in which I watched a drama because of actual cast was for Ji Sung, Jung Kyung Ho and Lee Sang Yoon. I started getting interested in Ji Sung's roles after Kill Me, Heal Me. Unfortunately, I wasn't a huge fan of the Entertainer or Protect the Boss, so I may just stop looking. I also wanted to see Jung Kyung Ho in a different role after watching Beating Again. I thought he was a terrific actor. When it comes to Lee Sang Yoon, I kind of liked him in Angel Eyes despite totally hating the drama. I wanted to see this actor in something I would feel less hateful towards. That's why I decided to give On the was to the airport a try. 
I agree with Linus...I normally choose dramas based on genr...Mainly romance or Sci Fi
I only watched one drama for an actress and that was Moon Lovers Scarlet Heart Ryeo for I.U.....Not really because I was a fan of I.U but because watching all the hot princes fall for a little girl,I was intrigued xD....I didn't even know I.U at all let alone that she was a singer...Never been a fan of kpop....But yeah she drew me in,her along with LJG:D
My sassy girl for Oh yeon seo but ended liking more Joo Won
Playful Kiss for hyun joong
Master's Sun for Ji Sub (do not regret this one in the least bit)
Shopping King Louis for Nam Ji-hyun     and
Oh My Ghostess for Park Bo Young
LOTBS for Lee Min Ho
gonna watch While you were sleeping for Lee Jong Suk and Strong Woman DBS for Jang Mi Kwan (I really liked him in Manhole)