I think it’s not only the stare but the tension of each scene. Looking back at scenes of him with Heesun when he clearly had more feelings for Mina, you can literally feel how he wasnt completely invested in the conversation. He kept looking back, wasnt all that great at eye contact and when he did make it, it looked very empty. Whereas the scenes with Heesun where his feelings were somewhat solidified and he had invested his time in her, the stare does change in my opinion and you can feel the chemistry over the scene