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 and which is able to evoke genuine emotions other than laughter.[2][3] 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 “small 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.”[4] 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 in the 21st century have been presented in one act.
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.”[5] 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.”[6] 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 and Hamilton) 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.
The above content comes from：https://en.wikipedia.org/wiki/Musical_theatre