Everyone was talking about IU and Baehyun's acting but in the later episodes IU's character gets less awkward and you (well I) felt more comfortable watching it compared to the earlier episode where it seemed like she was trying to hard to be quirky and cool.
Then with Baekhyun I didn't like his acting too much but I felt that is how they (the people behind the show) wanted his character to be portrayed so I let it slide because he's cute lol. But he really surprised and amazed me when he was talking to Haesoo about how she knew he liked her and everything. I felt like he did so well at acting out the feelings of an immature person with an unrequited love, at that scene I was just like "awwwwwwwwwwwwwwwww he likes her so much!!! SO. DAMN. CUTE!" lol that may have just been me though because I love crushes in dramas >.<
But to get back on track I think IU and Baekhyun are actually pretty good in this drama.
The one who I thought shouldn't even be a character in the drama let alone have a love line is Seohyun (not sure if I spelled it right) from SNSD. I was hoping I would like her character from the first time she appeared because my little sisters are big fans of her and her group and I didn't want one of their favs to suck in my eyes but I didn't like her. She just didn't seem to fit well with the mood and tone of the drama and it's characters. Then her chemistry with Nam Joo Hyuk. I just didn't see it, like I am unable to picture them together and get feels from it.
Well those are just my opinions and I wanted to see if anyone else had stuff to say about how well the cast is doing in their eyes or anything else in the drama.
K-Drama 
Then with Baekhyun I didn't like his acting too much but I felt that is how they (the people behind the show) wanted his character to be portrayed so I let it slide because he's cute lol. But he really surprised and amazed me when he was talking to Haesoo about how she knew he liked her and everything. I felt like he did so well at acting out the feelings of an immature person with an unrequited love, at that scene I was just like "awwwwwwwwwwwwwwwww he likes her so much!!! SO. DAMN. CUTE!" lol that may have just been me though because I love crushes in dramas >.<
But to get back on track I think IU and Baekhyun are actually pretty good in this drama.
The one who I thought shouldn't even be a character in the drama let alone have a love line is Seohyun (not sure if I spelled it right) from SNSD. I was hoping I would like her character from the first time she appeared because my little sisters are big fans of her and her group and I didn't want one of their favs to suck in my eyes but I didn't like her. She just didn't seem to fit well with the mood and tone of the drama and it's characters. Then her chemistry with Nam Joo Hyuk. I just didn't see it, like I am unable to picture them together and get feels from it.
Well those are just my opinions and I wanted to see if anyone else had stuff to say about how well the cast is doing in their eyes or anything else in the drama.
*not sure if there are spoilers but just in case