Incidental music
Incidental music dates as far back as Ancient Greece.  It was used in the 16th and 17th centuries, especially in the plays of Shakespeare whose characters often sing songs.  At this time incidental music was used more often for comedies than for tragedies.  Tragedies were about important people who were busy ruling their country and having wars.  In the plays they spoke in verse and were too noble to sing.  People thought that comedies were more suitable for including music.  The songs were often sung by actors playing the parts of shepherds, nymphs or clowns.