And this forces you as a designer to start telling the story of the interface. See it as a person, talking to the user… And like a storyteller you should define the character of the product. Is it agressive? Your friend? Your tutor or advisor? Will it talk to you as a restless child or be like Buddha himself? And what does it tell you? When? How?
An interesting approach, taking a photo of your sketch and using it in your wireframing/prototyping tool. I wonder what happens if you need a full-blown spec. I'm afraid it's not really scalable.