Okay...some music theater basics:
Musical theater is a broad term for theatrical performances that feature music as a central component and combines songs, spoken dialogue, acting and dance. While some musical theater composers create shows that incorporate other influences and musical traditions, Broadway composers tend to share a common musical influence of American musical theater.
So...a music theater composer does what?
At a high-level, a musical theater composer is responsible for a show's music. Most musical theater composers work with a development team that includes a book writer and lyricist who are responsible for the show's story, writing the script, and adding the lyrics. There are some versatile composers like Lin-Manuel Miranda, who take the creative process a step further by overseeing every aspect of a musical from start to finish.
In the "production stage" of a new show the composer will some times works with vocalists and the music director to tweak the final performance based on the talent and physical setting for the show, but the music director is usually permitted to make adjustments to the score based on a singer's "sweet spot", the configuration of the ensemble or issues associated to blocking and/or choreography.
What does composer-led musical theater mean?
When a composer decides much of the elements of the text, staging and design it is considered to be "composer-led". This is often a result of the composer formulating the basic milestones for story and identifying unique musical characteristic for each character during the early stages of the composition process, before identifying the book writer and the other members of the creative team. Examples of key music theatre artists who compose and direct their works include Georges Aperghis, Heiner Goebbels and of course, April Alsup. She, like other notable composers; Laurie Anderson, Meredith Monk and Robert Ashley, often perform their own works too.
Is there a "right" or "wrong" way to write a new music theater work?
No! When it comes to writing music theater works, there is so much variation. Some musical theater composers just sketch out songs with chords, rhythm and a melody, leaving the instrumentation to an orchestrator. Others might deliver a fully orchestrated score with a specific ensemble and voices. Musical theater composers create music for all sorts of purposes, instrumental compositions might be used for underscoring or to enhance scene changes, highlight an onstage event, or support a dance number. Vocal songs often advance the plot, supplement story threads, or show a characters' emotions, struggles, and thoughts.
There are so many examples of successful musical theater collaboration teams and the development process is often very different from team to team. The important thing to remember is to find the formula that works best for you and to enjoy the process.