I listened to it last night, and it is fairly subtle, but I'm surprised I never noticed it before.
The song is going along, and the music stops just past the halfway point. Christine says "I can't hear the piano well." Then Stevie says, kind of calmly, "Well, I can't hear any harmony, and it's probably because you're not singing."
Chris ignores the statement and continues to work on the sound of the piano.