I'm not sure where people got this opinion. The beginning of the movie was more grounded in reality, and you can clearly see the idol, Young Woo starting to get more interested in learning about acting, wanting to join Jae Ha in his "discovery" phase. I didn't see the idol challenge Jae Ha about who was "better" or not. If anything, the idol was challenging Jae Ha to confront his real feelings and growing attraction.
I think it's debatable if Young Woo was acting or not. He seemed to really dive into "method" acting and was acting even off the stage.
In the end, though, it was clear that Jae Ha was slipping--from literally slipping up and blending real life with fiction and using his girlfriend/wife's name instead of Claire, to his hesitation on stage.
I like what you said, that they weren't really acting and have to pretend to society. I see Young Woon as a more willing participant in playing the game--causing drama to promote the play, putting his reputation on the line for art. He clearly didn't care what happened--so if he really had feelings for Jae Ha, they seem more sincere.
But sadly, in the end Jae Ha retreated back to his former life. I'm convinced, though, that they are changed inside. It's a matter of time before your true self comes out somehow. Would be nice to see a time skip epilogue.
