Definitions and scope
Since the 20th century, the "book musical" has been defined as a musical play where songs and dances are fully integrated into a well-made story with serious dramatic goals that is able to evoke genuine emotions other than laughter. The three main components of a book musical are its music, lyrics and book. The book or script of a musical refers to the story, character development and dramatic structure, including the spoken dialogue and stage directions, but it can also refer to the dialogue and lyrics together, which are sometimes referred to as the libretto (Italian for "little book"). The music and lyrics together form the score of a musical and include songs, incidental music and musical scenes, which are "theatrical sequence[s] set to music, often combining song with spoken dialogue." The interpretation of a musical is the responsibility of its creative team, which includes a director, a musical director, usually a choreographer and sometimes an orchestrator. A musical's production is also creatively characterized by technical aspects, such as set design, costumes, stage properties (props), lighting and sound. The creative team, designs and interpretations generally change from the original production to succeeding productions. Some production elements, however, may be retained from the original production, for example, Bob Fosse's choreography in Chicago.
There is no fixed length for a musical. While it can range from a short one-act entertainment to several acts and several hours in length (or even a multi-evening presentation), most musicals range from one and a half to three hours. Musicals are usually presented in two acts, with one short intermission, and the first act is frequently longer than the second. The first act generally introduces nearly all of the characters and most of the music and often ends with the introduction of a dramatic conflict or plot complication while the second act may introduce a few new songs but usually contains reprises of important musical themes and resolves the conflict or complication. A book musical is usually built around four to six main theme tunes that are reprised later in the show, although it sometimes consists of a series of songs not directly musically related. Spoken dialogue is generally interspersed between musical numbers, although "sung dialogue" or recitative may be used, especially in so-called "sung-through" musicals such as Jesus Christ Superstar, Falsettos, Les Misérables, Evita and Hamilton. Several shorter musicals on Broadway and in the West End have been presented in one act in recent decades.
Moments of greatest dramatic intensity in a book musical are often performed in song. Proverbially, "when the emotion becomes too strong for speech, you sing; when it becomes too strong for song, you dance." In a book musical, a song is ideally crafted to suit the character (or characters) and their situation within the story; although there have been times in the history of the musical (e.g. from the 1890s to the 1920s) when this integration between music and story has been tenuous. As The New York Times critic Ben Brantley described the ideal of song in theatre when reviewing the 2008 revival of Gypsy: "There is no separation at all between song and character, which is what happens in those uncommon moments when musicals reach upward to achieve their ideal reasons to be." Typically, many fewer words are sung in a five-minute song than are spoken in a five-minute block of dialogue. Therefore, there is less time to develop drama in a musical than in a straight play of equivalent length, since a musical usually devotes more time to music than to dialogue. Within the compressed nature of a musical, the writers must develop the characters and the plot.
The material presented in a musical may be original, or it may be adapted from novels (Wicked and Man of La Mancha), plays (Hello, Dolly! and Carousel), classic legends (Camelot), historical events (Evita) or films (The Producers and Billy Elliot). On the other hand, many successful musical theatre works have been adapted for musical films, such as West Side Story, My Fair Lady, The Sound of Music, Oliver! and Chicago.
Comparisons with opera
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. Some musicals, however, are entirely accompanied and sung-through, while some operas, such as Die Zauberflöte, and most operettas, have some unaccompanied dialogue. Second, musicals also 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.
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.
Some works (e.g. by George Gershwin, Leonard Bernstein and Stephen Sondheim) have been made into both "musical theatre" and "operatic" productions. 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. 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." 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".
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, and some musicals may call for only a piano or two instruments. 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." Musicals may begin with an overture played by the orchestra that "weav[es] together excerpts of the score's famous melodies."
Eastern traditions and other forms
There are various Eastern traditions of theatre that include music, such as Chinese opera, Taiwanese opera, Japanese Noh and Indian musical theatre, including Sanskrit drama, Indian classical dance, Parsi theatre and Yakshagana. India has, since the 20th century, produced numerous musical films, referred to as "Bollywood" musicals, and in Japan a series of 2.5D musicals based on popular anime and manga comics has developed in recent decades.
Shorter or simplified "junior" versions of many musicals are available for schools and youth groups, and very short works created or adapted for performance by children are sometimes called minimusicals.