The follow-up question is where interviews are won

Why the second question matters more

Your first answer is the one you prepared. The follow-up is the interviewer checking it. "How did you know that was the cause?" "What would you do differently?" "What did your manager say?" They are not trying to catch you out. They are trying to tell the difference between someone who did the work and someone who has heard it described.

An answer that sounds polished but cannot survive one more question does more harm than a plainer answer with real detail behind it.

The questions that usually come next

You can predict most follow-ups from your own answer. Listen to it and ask:

• How did you know? Anything you say you found, noticed or decided invites a question about how.
• What was your part? If you said "we", expect to be asked what you did yourself.
• How did it turn out? If you did not say, they will ask. If you do not know, say so plainly and say what you would look at to find out.
• What would you do differently? Have one honest thing ready. "Nothing" is rarely believed.

How to practise it

Say your answer out loud, not in your head; it is the only way to hear where it is vague. Then have someone ask "how did you know?" or "what happened next?" and answer again without notes. If you are on your own, write down the follow-up you would ask if you were the interviewer, and answer that.

Then answer the original question a second time, starting from what you said, not from a fresh script. The second attempt is usually tighter, and it is still yours.

When you do not know the answer

Say so, then say how you would find out. "I didn't track that at the time. If I were doing it again I'd look at the callback numbers before and after." That is a strong answer. A made-up number is not, because the next question will be about it.