Working Around Songs and Source Music
Many movies nowadays don’t only contain score music but also songs that are usually not written by the score composer but licensed. Also, the places where they are used in the movie are usually already fixed before the composer begins to write the score.
Nevertheless, it is important for you as a composer to make your music work with the songs. Especially where score and songs are used very close together in the movie, it is important to take care of proper transitions or at least avoid unwanted contrasts. This means, that the transitions should ideally fit harmonically but also emotionally. Jumping from an action score without any preparation into a melancholic ballad will feel very strange. So even when you don’t work on the songs and you don’t really have a lot to do with the decision about them, make your music work with them.
The same applies for any source music (music that is being played on screen) that is near a score cue. Unless a hard contrast is desired, you should try to treat any “external” music as part of your score and fit it into your musical surrounding as well as possible.