When to Test Your Prototype

Rough drafts are the staple of the creative business. Art directors sketch and storyboard. Interaction designers mock and prototype.  

How soon should you put it in front of people?

On the one hand, here's an IDEO designer with a post "Why You Should Start Prototyping—Right Now":

Instead of starting with our usual process where we gather inspiration, we thought: What if we share a prototype we are really excited about with the client right away?

A head of design at Atlassian invokes Jony Ive:

"[The design process] is about designing and prototyping and making. When you separate those, the final result suffers."

And continues:

Just like a picture is worth a thousand words, a prototype is worth a thousand user stories.

On the other hand, Alan Cooper cautions in a Twitter thread:

Prototyping and testing is not interaction design. It helps, but it isn’t user centered. It’s designer centered. Prototyping & testing has one huge virtue: it makes management happy because it never rocks the boat. It never requires big changes. It always keeps the designer in command. It’s very ego gratifying.

When you put an artifact in front of a user, you instantly shut down an infinity of good ideas, avenues of thought, opportunities to create. 

The first prototype you show sets the direction for everything that follows. It becomes the reference point. Put the first button in the wrong hole, and you'll will mess up the rest.