The follow-up decides the round, not the answer

Most candidates prepare answers. Definitions, explanations, the correct description of how something works. Then they deliver one, the interviewer nods, and asks the question that actually decides the round.

Why that approach. Where does it break. What would you do at ten times the traffic. What did you consider and reject.

That second question is the assessment. The first was a way of getting to it.

I have watched a lot of engineers give a textbook-perfect first answer and then have nothing underneath it, because they memorised a conclusion without the reasoning that produces it. The moment that becomes visible, the level assessment moves down, because reciting a conclusion is what a mid-level engineer does and defending a trade-off is what a senior one does.

The tell is in your notes. If your preparation contains answers but no rejected alternatives, you are preparing recall rather than judgment. Real reasoning always has something you decided against.

The fix, and it takes an evening.

Take the three topics most likely to come up for your next round, the three you feel weakest on. For each, write four things rather than one.

The short answer, in a sentence.

The production answer, with the trade-off named.

One way the approach breaks.

The follow-up question you would ask if you were the interviewer.

Then answer your own follow-up. If answering it produces a restatement rather than new information, you have found a topic you know as a definition and not as an experience. That is the one to go and read about properly.

Three topics prepared this way beat seven prepared as answers. Every time, and it is not close.

The interview does not start when they ask the question. It starts when they challenge your answer.