Recently, 'The Penthouse' scriptwriter Kim Soon-ok sat down for an interview where she talked more about the drama.
During the interview, Kim Soon-ok picked the ending scene of the episode 15 as one of the best scenes from the drama.

She continued, "Kim So Yeon wanted to bring out the emotions as much as possible so she practiced piano for months. She performed the piano herself in the scene."
She added, "I sincerely applauded her passion. I was particularly excited for the ending of episode 15 when I was writing the whole script. Kim So Yeon did such a great job that as the scriptwriter, I feel like I was carried by her amazing performance."

(Credit= SBS The Penthouse: War in Life, 'SBS Drama' YouTube)
(SBS Star)