Some Broadway shows are like operas in that they use what we call a "through composed" score. That means that the music continues throughout the show --- as accompaniment to the singing as well as providing emotional underpinning for the dialogue (if there is any that is unsung) and the action sequences. This also means that the timing of everything needs to be precise.
As I watched a documentary of "The Music of John Williams," I learned that movie makers like Steven Spielberg usually put cuts of sample music under their previews of scenes to give an idea to composer of what they want musically. Apparently, John Williams didn't like that. He preferred getting his own ideas for underscoring a new movie. And aren't we glad he didn't just follow the director's ideas? We might never have had the incredible scores of the Star Wars or Indiana Jones movies.

Then there is the problem of TIMING. Sometimes these little actors get stage fright and forget their lines, or don't deliver their lines quickly enough, or need prompting for cues, and the list goes on. If they are doing the show to a pre-recorded through-composed score, there is a good chance that they won't get the timing right.
I've gone to see several through-composed shows done at the Junior High School level, and even with older kids I was so disappointed. Not only do the kids need very good AMPLIFICATION, they also need to be BLOCKED in such a way that the poor audience can figure out which characters are singing or delivering lines. When all the voices are in Treble Clef, it is extremely hard to differentiate who's who in a cast.
For my new show, "No Time for Monkey Business," the introduction of 10 brand new songs happens mostly in the first two thirds of the show. That is plenty of new music for an audience to digest. So, I've decided to use parts of these songs as dances and LEITMOTIFS (thematic elements) to underscore some of the drama in the show, especially as it relates to particular characters.
For example this "Jungle Theme" will be used as:
1. the introduction to the show
2. a song about what to do after the death of the old King
3. mysterious mood music
Now, I am going back to building costume pieces...