Quick answer
Prototype the riskiest interactions in code early — the core flow, data-heavy screens and signature motion — so decisions are based on how the product actually feels, not just how it looks.
Design tools are excellent at answering "what should this look like?" They are much worse at answering "will this feel right when it is alive?" Motion, loading, real content length and device performance only show up when something actually runs.
Prototype the parts you are least sure about
We do not rewrite the whole product before design is approved. We build the three or four interactions that carry the most risk: the core flow, the data-heavy screen, the transition that makes or breaks the experience.
- A real form with real validation and error states
- A list with messy, production-shaped data
- The signature interaction, running on a mid-range phone
It makes critique more honest
Stakeholders react differently to a running prototype. Comments move from "I do not like this colour" to "I got lost here" — which is the feedback that actually improves the product.
The goal is not to skip design. It is to spend design effort where it survives contact with reality.