I caught a BBC podcast on using a narrative to describe and promote policy: Jackanory Politics. There is a fascinating quote from Robert Maxwell describing a story as:
a fact wrapped in an emotion that compels an action that produces a change.
As Maxwell explains, a compelling story has five elements:

1. the compassion with which you tell the story
2. a hero that the audience can relate to
3. an antagonist or obstacle that must be overcome
4. the moment of awareness where the hero realises the solution
5. the consequent impact of the change

Number 4 - the moment of awareness - is typically ignored, Maxwell warns, but the best story-tellers apply this technique to great effect.

And all this time I was convinced that policy relied on evidence; apparently that's only half the story...