(This is an old blog entry from about four years ago.)
The development of musical theatre has always been entwined with the medium of commercial popular music - Tin Pan Alley standards, swing and pop, commercial rock and roll, etc.
But to my mind, there has always been a tug of war in musicals - between the need to create a strong play and then need to put forth strong songs that stand on their own.
Some musicals let the play rule: the songs have unusual structures, can be full of dialog and recitative, can have odd and irregular rhyme schemes and no button at the end to trigger applause. Other musicals let the song rule: these songs usually have traditional structures, pop music styles and often lyrics that are general enough to work outside the confines of the play. (It should be noted that operas have the same polarities - scene versus aria. I think the works of Verdi span one end of the spectrum to the other as he matured.)
Most musicals are somewhere in the middle, between these two poles. And there is certainly no singular solution. Michael John LaChiusa's work definitely tends towards the play end of the spectrum, subverting song structure to serve the scene at hand. Older book musicals from the Thirties, and the earlier shows of Lloyd Weber are examples of the songs ruling the structure of the play. The Rodgers and Hammerstein canon still lets the song rule (to my mind), but the songs now are made to assist the play's forward motion.
This is all obvious to anyone working on musicals. But the orchestrater has a special role in this polarity. His approach to the song can push it more towards one end or the other of the spectrum. He can emphasize the 'popness' of a song, and isolate it from the play that's going on. An example of this was the song "Wanting" from the musical "Rags". It was definitely written to also have a life outside of the show, the music sounding very contemporary and the lyrics having no specific reference to characters in the play. The orchestrater's job was to deliver the commercial viability of the song to the listener, while simultaneously integrating it into a ragtime/jazz score set in the early 20th century.
An interesting twist on this polarity is the work of William Finn. Finn delivers a series of songs, each asking to be treated as strong individual units. But he also works to create a play FROM the series of song moments (as opposed to a series of scenes containing songs). He will open up the song structure to allow book material to be sung, but never abandons the song as the building block for the play. Here, the orchestrater needs to button and catch the varied song styles, but never lose thread of the play and characters that is winding its way through the piece as a whole.
Sometimes I think Finn's work is influenced by the record album as a literary form. Many albums made in the Sixties and Seventies were fully integrated, cohesive works. Again, a series of individual songs adding up to a greater whole.