In “Indiana Jones and the Last Crusade” there is a scene where Indy and his father go the get the diary back and, in the process, meet up with Hitler.
That scenes begins with the score functioning as source music. John Williams wrote music that an unseen brass band would be playing in the square as the Nazis burned books.
As Indy gets pushed to the center and meets up with Hitler, the music becomes underscore by performing an ominous drum rhythm only, letting the brass drop out for a while as the tension plays out.
When Hitler picks up the diary and signs it, we realize all is well and the military march resumes! There is, of course, no way that the military band present in the square would have done this! So the score here has actually switched roles and become underscore.
It starts up again with the same melodic material as the source music, but almost immediately cadences on the scene transition, the way underscore does!
So this musical sequence blurs the line between source and score, something I recently did as well in a score I wrote, which is why I thought I would bring this up. Something good for directors to be aware of;
Music in a scene can function as both source and underscore!