Scott Berkun, "Strategies of influence in interaction design," UIWeb.com, Nov 2001

A prototype of your new search page design reflects a set of design choices you think should be made. But what are the business impacts of those choices? How long will it take to build? How will it effect advertising rates, or partnerships? What code changes are needed to make it possible? In the abstract, some designers feel these are not design considerations, and instead are just matters of implementation for someone else to figure out. This is a trap. While it’s great to ignore constraints to inspire creative thinking, if your want your work to reach people’s web browsers or desktops, you have to plan to involve yourself in the practicalities of realizing your ideas. The more skilled you are at assessing those aspects of a design, the more welcome you’ll be to participate in the process.
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The two most powerful forces of influence in the interaction designers toolbox are the video and the prototype. Humans are visceral creatures. We respond to things that call on our senses. Specs, code, and even bitmaps are all static, limited attempts to represent what the experience of interaction will be like. It takes an inventive imagination to read these things and accurately visualize anything. It’s typically only those few individuals with practiced imaginations, regardless of job title, that can do this well. If you need to convince someone that the current design has problems, you must show them... If you want to get people’s attention around your work and the user experience, there is no better way.

The prototype offers you another form of direct influence. It’s almost a secret weapon. While everyone in the room is debating in front of the whiteboard about what the design should be, or how it should behave, you can show the group a real live working example of your ideas . They can even interact with it depending on what you’ve done. It’s a huge advantage.