I don't think that the two main actors did a bad job, but I don't think they did an amazing job, either. Suzy has the same expression when she cries, and Lee Jon Suck is always doing that weird smile thing that's supposed to be endearing but it's just awkward. I did like his scenes with his brother, though, and the inspector and Woo Tak. Regarding their acting as a romantic couple, it felt forced and awkward and they barely touched and it felt so weird to watch.
I think Jung Hae In as Won Tak and Lee Sang Yeob as Yu Beom were the ones who stole the show for me. I think they completely stole your attention from everybody else when they were on the screen, and did a better job of showing different emotions and sometimes even make you empathize a little with them (in Yu Beom's case, because at first I thought they'd just leave it at "he's a jerk who told Jae Chan to fake his notes and would've blamed Hong Ju" and then show him becoming a better person, and while I respect their decition to not redeem him, I kinda hoped he would, at some point, because the actor did such a good job of showing his own internal conflict).
The music was sort of meh for me.