Good casting. IU can really pull off any genre be it on music or drama/film. No wonder she received a baeksang nomination for best actress.
Not only she slay in fashion but the emotions she shows is really outstanding. IU did not only used her face for being beautiful but damn it, her facial expression is the best. Especially her eyes, how she manages to have her tears pool on her eyes is magic. There is a particular scene is which her emotions changes from fear, determined, relief, confusion then anger all in which happened without blinking. Just watch it and you will know!!!
And oh, you will cry on this. Like ugly cry.