The song itself is like the plot. It has its own feel and purpose and sound. It has its own beginning, middle, and end. The average listener can listen to the song as a whole (without focusing on individual characters) and thoroughly enjoy it.
Even though the song seems to be its own entity, it's sound is almost entirely influenced by the instruments. You could take the same song and get an entirely different sound depending on which instruments you chose to play it. Imagine Moonlight Sonata being played by a cello...or by a banjo...or by a xylophone. Obviously the three would sound different. So it is that your characters influence your plot. Their reactions are unique to themselves.
Your characters each have their own story to tell -- just as each instrument in the concert has its own piece to play. As the composer, you must make sure you give each instrument the assignment they need to add their voice to the music -- enough to add interest to the song but not so much that they drown out the music you want the audience to hear overall.
with multiple instruments
each carefully chosen
each given their own part to play
to create the music that you wish the audience to hear.