Your risky assumption is like a fence you need to see over.
The goal of a prototype, or an MVP is to climb the smallest possible hill in order to see over that fence.
It doesn’t make sense to climb Everest (build the entire app) to see over a small fence. You just need to do the cheapest and easiest thing that will test your big assumption about a project.
Four girls at UD wanted to start an app that allowed girls to swap clothing locally. They thought it was a good concept, but no one knew if other people would like the idea. They identified their riskiest assumption as being unsure if people were comfortable with the idea of swapping clothing.
Did they build an app to test this assumption? NO! Apps are expensive, time consuming and hard to get people to download. Before they built an app, they held a live event that simulated the action of swapping in real-time. They’ve now hosted two of these events and the attendees have loved them.
Through that cheap and easy “prototype” they have de-risked the business in one important area. They also now have a captive audience of 100 girls that would love to download the app the day it comes out. Sounds like a much better plan that launching an unproven idea to no audience.
Starting small works wonders. Climb the little hill first.