Paper prototyping is a common usability technique to quickly test out an interaction before expending too much effort on programming or designing. The value in paper prototyping is that with extremely low effort, you can test the interaction rather than the appearance of an interface.
I just came across a great iOS app that lets you add some real interactivity to your paper prototypes: POP Prototyping on Paper. You simply sketch out your screen, take a picture using your iPhone camera, and then add interactivity. It’s a brilliantly simple idea.