Musical theatre is closely related to the theatrical form of opera, but the two are usually distinguished by weighing a number of factors. First, musicals generally have a greater focus on spoken dialogue.[7] Some musicals, however, are entirely accompanied and sung-through, while some operas, such as Die Zauberflöte, and most operettas, have some unaccompanied dialogue.[7] Second, musicals usually include more dancing as an essential part of the storytelling, particularly by the principal performers as well as the chorus. Third, musicals often use various genres of popular music or at least popular singing and musical styles.[8]
Finally, musicals usually avoid certain operatic conventions. In particular, a musical is almost always performed in the language of its audience. Musicals produced on Broadway or in the West End, for instance, are invariably sung in English, even if they were originally written in another language. While an opera singer is primarily a singer and only secondarily an actor (and rarely needs to dance), a musical theatre performer is often an actor first but must also be a singer and dancer. Someone who is equally accomplished at all three is referred to as a “triple threat”. Composers of music for musicals often consider the vocal demands of roles with musical theatre performers in mind. Today, large theatres that stage musicals generally use microphones and amplification of the actors’ singing voices in a way that would generally be disapproved of in an operatic context.[9]
Some works, including those by George Gershwin, Leonard Bernstein and Stephen Sondheim, have been made into both musical theatre and operatic productions.[10][11] Similarly, some older operettas or light operas (such as The Pirates of Penzance by Gilbert and Sullivan) have been produced in modern adaptations that treat them as musicals. For some works, production styles are almost as important as the work’s musical or dramatic content in defining into which art form the piece falls.[12] Sondheim said, “I really think that when something plays Broadway it’s a musical, and when it plays in an opera house it’s opera. That’s it. It’s the terrain, the countryside, the expectations of the audience that make it one thing or another.”[13] There remains an overlap in form between lighter operatic forms and more musically complex or ambitious musicals. In practice, it is often difficult to distinguish among the various kinds of musical theatre, including “musical play”, “musical comedy”, “operetta” and “light opera”.[14]
Like opera, the singing in musical theatre is generally accompanied by an instrumental ensemble called a pit orchestra, located in a lowered area in front of the stage. While opera typically uses a conventional symphony orchestra, musicals are generally orchestrated for ensembles ranging from 27 players down to only a few players. Rock musicals usually employ a small group of mostly rock instruments,[15] and some musicals may call for only a piano or two instruments.[16] The music in musicals uses a range of “styles and influences including operetta, classical techniques, folk music, jazz [and] local or historical styles [that] are appropriate to the setting.”[4] Musicals may begin with an overture played by the orchestra that “weav[es] together excerpts of the score’s famous melodies.”
The above content comes from：https://en.wikipedia.org/wiki/Musical_theatre