i know this is veryyyyy late and i usually never comment when the posts are old but i just have to hear cause AHHHHHHHHHHHH!!!! and girl i totally get you. i also didn't know how this would work but it works sooooo well. i also think a huge contributing factor is that Choi HyunWook is very charismatic in his presence whether on screen or ofscreen. he has this effortless and nonchalant aura about him that gives him an advantage on appearing very natural on screen despite the nature of the role, like thats just how he is and he is not putting in effort to be this way so it never looks like he is acting (unless ofcourse the character is supposed to put on an act). also, he is very calm and despite have a shy, awkward cute personality, still retains a brooding quality (possibly from being an athlete for around a decade). so despite him 'looking' young, still gives him a mature aura, like he has his shit together, and you can depend on him to lead you (if you see him interviews or just videos of him out and about, he really does give that vibe). all of this, just falls into place perfectly for this role cause he is supposed to be really young so will have these child like innocent moment against an older person but like you said in an earlier episode 'he is a child who had to grow up too quickly'. he is emotionally mature in that sense cause of his life experience and ofcourse very good at his work cause 'he wants to be acknowledged by his grandma so has been working very hard' and ofcourse makes sense he is a directly at this age cause ofcouse he didn't go through the normal sort of promotion route. the heir training even in real life start pretty early.