is to think about their motivation for telling the story, and who their ideal implied reader is.

Yes, exactly. I knew exactly why one of my narrators in WTQD was telling her story, and she even tells the reader why she's writing. But one of the many things which made the second narrator difficult was that I didn't really know why she'd be telling herself all this, because it didn't suit her character to do anything of the kind. And, actually, my agent spotted it before I did, and after that it was easy. Well, it wasn't, but it was a lot easier, because I knew what the drive of her need to tell was, and that conditioned and awful lot of what she said and how she said it...

Emma

<Added>

I mean, my agent spotted who this narrator could be talking to, and then I worked out why she would be, and then I was off...