You know that part in movies where the main character turns on their car radio and the song that’s playing slowly fades in and becomes the movies background music? I like that
I love the opposite, where the background music is seemingly just background music until the cut to someone turning off the radio and the music abruptly cuts off.
The first one is called the Diegetic Switch. The second one is the Left the Background Music On trope which is more fun and full of comedic potential