Oh, and a follow-up, WAV, in response to your observation about pub sessions: yes, many of the musicians are playing the tune most of the time, but some instruments include chords and drones that provide a harmony (guitar, mandolin, melodeon, accordion, some concertina playing, some pipes) so there will be quite a lot going on apart from the melody. If the melody is a simple one - say, a slowish waltz - you'll often hear improvised harmonies on recorder/whistle/fiddle or other melody instruments.