Having written my first musical, DRAGON'S BLOOD (a 2.5 hr work, with 24 musical numbers in it), I have formed a few thoughts in this area:
The Difference Between Musicals and Operas
I've looked at a lot of comments on the web (e.g. operas have "high-brow" music and themes, while musical theater is more "low-brow".) However, there are many exceptions to most distinguishing characteristics people come up with.
My criterion for distinguishing is simple: If the dialog is sung, then it is an opera. If the dialog is spoken, then it is a musical.
Personally, I find that the melodic lines sung during the dialog segments of many operas are not on par with the songs and so I prefer dialog to be spoken and I prefer songs to be self-contained.
There are Two Different Ways To Write Musicals:
The standard method is: Generate the characters and the plot and then write all of the songs to fit them.
An alternative method is: First write some songs that could fit into a plot schema (a more general structure capable of producing a variety of more specific plots) and then modify both the song lyrics and these potential plots variants so that, over time, they conform to each another's constraints.
I used this alternative method in writing Dragon's Blood: I took a general plot schema (about a vampire wanting to become human and a human wanting to become a vampire) and some songs I had already written (e.g., of a vampire offering a human woman a concoction that will turn her into a vampire) and then I modified song lyrics to fit the developing characters and plot structure. I think that the advantage of this method is that the resulting lyrics will conform to the musical only just enough to make sense in the musical's context. The result, I believe, is that the lyrics will be more universal and thus potentially more meaningful to people who do not care about the musical itself. The danger of the standard method is that the songs that are written will tend to be overly specific to that musical's characters and plots and, therefore, less universal.
Here is a hypothetical example of what I am talking about: Suppose we have a scene in some musical, in which the hero (David) is trying to get the heroine (Davina) to go skating with him. The song composer who employs the standard method might end up writing lyrics of the sort: "Davina, I know that your skating will be divine. If you skate with me you'll have a real fun time."
In contrast, the song writer with a plot schema will tend to produce lyrics that are more general: "Together we can feel the divine. Together we can become intertwined." Once the song is written, if the plot develops into containing an actual skating scene, then the lyrics can be modified, but with the goal of generality maintained. For example, the phrase "we're skating on thin ice" could be used for both a skating scene and an unrelated scene that involves some form of danger (physical or in a relationship).