For Jae Ha.. he was frustrated professional actor who didn't like someone who's slacking off. He needed and wanted to get Young Wo into his role. But, during the process he fell for him. With his little glances and body language it was obvious he was driven to Young Wo. Even his gf realized there was something going on. It's true, that she got mad because he was living as his character "again", meaning this wasn't the first time.
But, after rejecting her in bed, I think she realized this wasn't just method acting.
They both played themselves. They both played their own little games..but falling in love in between.
Their age gap was shown during the conference. YW, the youth and JH, the realist, the one who couldn't accept his own feelings, not even YW. He made himself believe it was just acting.
For the ending...Jae Ha decided to neglect his true self and feelings, going back to his life.
Young Wo, tho happy with himself, he realizes that he will never get his desired life. He won't be with a person he loves.
So even though their play was called 'Unchain'...their feelings were chained. Because that's the reality. Same sex relantioships are still not accepted by majority