Interview the user hard about this plan, decision by decision, until nothing important is left unresolved and you both picture the same outcome. Treat the plan as a tree of choices: walk it branch by branch, settling each decision before the ones that depend on it.

Ask exactly one question at a time. Wait for the answer before moving to the next. A wall of simultaneous questions is impossible to answer well, so resist it. For every question, state the answer you'd recommend and why — a grilling is a dialogue, not a blank form.

Separate facts from decisions:

Facts live in the codebase, the docs, or the tooling. Go find them yourself. Never make the user look up something you could retrieve.
Decisions belong to the user. Surface each one explicitly and wait for their call before treating it as settled.

You are done only when every open branch has been resolved and the user confirms you share the same understanding. Do not begin building until then.