Bollywood song and dance

Songs typically comment on the action taking place in the movie, in several ways. Sometimes, a song is worked into the plot, so that a character has a reason to sing; other times, a song is an externalisation of a character's thoughts, or presages an event that has not occurred yet in the plot of the movie. In this case, the event is almost always two characters falling in love.
Bollywood films have always used what are now called "item numbers". A physically attractive female character (the "item girl"), often completely unrelated to the main cast and plot of the film, performs a catchy song and dance number in the film. In older films, the "item number" may be performed by a courtesan (tawaif) dancing for a rich client or as part of a cabaret show. The dancer Helen was famous for her cabaret numbers. In modern films, item numbers may be inserted as discotheque sequences, dancing at celebrations, or as stage shows.
For the last few decades Bollywood producers have been releasing the film's soundtrack, as tapes or CDs, before the main movie release, hoping that the music will pull audiences into the cinema later. Often the soundtrack is more popular than the movie. In the last few years some producers have also been releasing music videos, usually featuring a song from the film. However, some promotional videos feature a song which is not included in the movie.