People break out in song and/or dance in public, and anonymous passersby join in-musical.
People break out in song and/or dance in public, someone gets hit by a truck, the police are called, and anonymous passersby yell “Shut the fuck up!”-not a musical.
There actually *was *a musical of “Saturday Night Fever” that played on Broadway from 1999-2000. That version has not yet been turned into a movie and the original film is clearly not a musical.