I started Tree With Deep Roots because of Jang Hyuk. I started to love him in beautiful mind, now Im binge watching all his drama and it was fantastic journey for me... :) Love him in Chuno, Fated to Love You, Flu. And he's so good in Tree with deep roots... :) Bansai! :) My favorite actor.. :)
That was the start for me...I watched everything with Jo In Sung and Jo Hyun Jae in it.
just few drama, price and prejudice because of Choi Jin Hyuk the main cast there
Depends, even if there is one of my fav, if the plot is not good enough i won't watch it.
For example Triangle, The girl who sees smells or three days etc ---> I like Yoochun and Jaejoong though
I like Lee Dong Wook too but I couldn't finish Blade man =(
K2 ... Because of Ji Chang Wook. 
yes,k2 becoz of Ji Chang Wook, scarlet heart ;reyo becoz of IU ,Oh My Ghostess becoz & best lee soon shin becoz of jo jung suk and many more ...I only watch drama if it has cute actoress and good looking hero like chang wook .
Pretty much I'm in for everything Seo In Guk or Lee Jun Ki related.
Angry Mom for Kim Hee Sun
Remember - War of the Son for Park Min Young (Dropped it - too much crying too often)
Moon Lovers & Pretty Man for IU which then made me watch Iljimae and Joeson Gunman and Arang and the Magistrate for Lee Jun Ki
Jackpot, Mary Stayed Out All Night, You're Beautiful for Jang Keun Suk
anything with Lee Moon Shik, anything with Kim Mi Kyung, anything with Kim Seul Gi - at least if it's terrible drama I'm guaranteed one good performance
City Hunter because I just watched Lee Min Ho in Faith
Falling for innocence because I just watched Jung Kyung Ho in Cruel City
The Kings Face, 38 Task Force, Shopping King Louis, I Remember You for Seo In Guk
Empress Ki for Ha Ji Won
Bachelor's Vegetable Store for Ji Chang Wook
Reply 1997 and Sassy Go Go for Jung Eun Ji

Basically, if i see an actor i like in a drama, i go down that actors rabbit hole and watch everything they were ever in.

The first drama I have watch with Lee Joon Gi in it was Time Between Dog and Wolf. I felt in love with him since then. So I watched lot of other Joon Gi's dramas only for him. I watched Iljimae even if I hated historical dramas and then Hero, two weeks. I was very happy to see him for like 30 seconds in she was pretty :P. I watched Scholar who walks at Night but I didn't enjoy it very much so I was less motived for Arang and the magistrate and Gunman in Joseon, because as I said I'm not fan of historical romance. I recently watched Moon lovers because of Lee Joon Gi, Ha Neul, Ji Soo, IU and Baekhyun. I was very happy to see a drama with all this great actors in it.
I rarely watch a drama without knowing any actor in it. I don't like to read synopsis because some of them spoil the story events so the cast is my only reference for choosing a drama to watch, plus popularity of course.
So I watched :
for Lee min ho in boys over flowers : I am Sam, Personal taste, Heirs, City hunter and Faith
for park shin hye in you're beautiful : Heartstrings, Flower boy next door, Heirs and Pinocchio
for woo bin in Heirs : School 2013, Uncontrollably fond, the film twenty but I couldn't watch more than 20 min
for hyun bin in secret garden : hyde, jekyll, me.
for Lee jung suk in I hear your voice : Secret garden, School 2013, Pinocchio and W

There are lot more like kim soo hyun, ja na ra, ji sub, Hwang Jung Eum, infinite members, Ji soo, Song Hye Kyo, Ji sung etc.

Something about 1% and Drinking Solo - Han Suk Jin
Doctors, Angry Mom, Sassy Go Go, One Way Trip - Ji Soo
Two Weeks - Lee Joon Gi
Oh My Venus, Always - So Ji Sub
Page Turner, Bring It On, Ghost - Kim So Hyun
oh @HaeSoJiSooTrash, we're similar!
I watched Angry Mom because of Kim Hee Sun and then Sassy go go & Page Tuner because of Ji Soo and then Bring it on ghost because of Kim So Hyun. I think I could totally link a lot of dramas I watch like that because I go on from one actor to another x)
Yes of course haha. I tried watching Heart to Heart, Heartless City, and Oh Hae Young Again because of Lee Jaeyoon hahaha. I watched a couple of eps but I couldn't finish it because I didn't like it lol.

I watched Bad Guy for Lee Jae Wook...but then I ended up liking Kim Nam Gil. Hen after that I watched Queen Seon Deok and finished the whole thing because of Kim Nam Gil...although he didn't appear until like ep 20 lol. Then I watched the boring drama shark because of him too.

I watched Incarnation of Money for Kang Jihwan but then this time I actually loved the drama (it became my favorite drama ever..so far). And I watched Hong Gil Dong too for him...but I actually liked this drama too.

I watched Six Flying Dragons for Byun Yohan..but then I stopped at ep 40ish.

I watched Beethoven Virus because for Jang Geun Suk..even though it was ehh.


Second To Last Love, Beacause of Kim Seul Gi, my little love
I may say now I am watching Warrior Baek Dong Soo mostly cuz Ji Chang Wook, but also cuz I started to be curious about historical dramas (and korean history). I am kinda sure there was at least 1 drama/movie I watched only cuz an actor but can't remember title now, have to think...