Star Trek: The Original Series - Amok Time
Look at Nurse Chapel’s face. She wants her smile to be radiant, but at the same time mixed into her eyes is such melancholy and even a flash of jealousy that Spock will never feel this way about her.
And that’s why I love her, AND Majel Barrett.
(I would swear Spock modifies his behavior on her behalf, more than McCoy’s.)