A clickable prototype can be as simple as hooking up a few wireframes together so that when users click on a button they move to another static mockup. It can be as complicated as building a whole user interface that behaves almost identically to what a user would see in a real product but doesn’t do anything on the backend. The closer you can get to the end product, the better the feedback you’ll get.
It's a trade-off between time/money spent and quality of feedback.
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