In DESIGNING INTERACTIONS , he talks about the three questions that an interaction designer has to answer during the process of designing interaction.
1. How do you do?
2. How do you feel?
3. How do you know?
The person that you are designing for does something. You design the affordances. When you are designing the way people do something, you have a choice between a handle or a button.
How do you feel?
How do you get feedback? You design the way that the machine, or the system gives feedback to the user or the books looks to the user or the sign communicates. Much of the emotions about the world come from the sensory qualities of those media that we present things with. Here we are in the realm of the surface quality: the color, the texture, the light.
How do you know?
It is important for a user to know what they are going to do with the product that they are coming in contact with. It is the role of the designer to help people to understand how to use a product by showing them a map or a path The map will show an overview of how the product will work and a path shows them what to do.
ADDITIONAL INFORMATION ON BILL VERPLANK:
Bill Verplank sketches metaphors
Interaction Design Lecture