A Hollywood musicals quiz is available on the Oxford University Press blog: link. [Link checked on 3-01-2024.] The quiz is based on a chapter in a book by Todd Berliner. The chapter "traces the history of the convention that characters in Hollywood musicals burst into song without realistic motivation."
This idea is related to—but is distinct from—the audio dissolve that we discuss in Hearing the Movies, chapter 3. Although we do say that the audio dissolve "serves as a transition to song and dance," we use the term as Rick Altman defines it, for the transition from diegetic song and accompaniment to song with a non-diegetic accompaniment. In such cases, the initial status is clearly diegetic ("The character seems to know very well that he or she is singing"). In fact, "one very common device in classical Hollywood musicals is to show one or more characters gathered around a piano." Once this is established, and "as the song progresses, nondiegetic orchestral accompaniment enters, replacing the piano."
Berliner is concerned, then, with what happens before the singing starts and with the transition into it. This notion that musicals permit "bursting into song without realistic motivation" is consistent with our generalization that film "musicals do not maintain the clear separation of diegetic and nondiegetic registers and so cannot rigorously enforce the boundaries of the diegetic world, which seems to constantly dissolve under the force of song." (Quotes are from Hearing the Movies, second edition, p. 69).