Dialogue can reveal character, advance plot, it's a great potential source for humour, and it can permit characters to discuss the central themes of your novel.
Damon Lindelof, one of the show's creators, says getting ABC to announce an end date for "Lost" three years in advance, gave them time to carefully craft the plot line leading to the finale.
VOA: standard.2010.05.25
By its nature, such a diffuse movement is much harder to monitor than a unitary organisation: witness the apparent lack of any advance intelligence about the Madrid plot.