How important is it to know a topic for an interview question?

Many people will just go with step 2, and that is just about the worst thing you can do, assuming that you actually do not guess correctly.

If it is a core element of the role you are interviewing for, very important.

But knowing every little aspect and nuance is NOT important. So if you are asked a question to which you do not know the answer, then the 3 step approach to answering it should be

  1. Say that you do not know;
  2. based on your current understanding of the topic about which you are being asked, explain what you *think* the answer probably is;
  3. then explain the process you would go through to investigate is that answer is correct, and how you would proceed if your guess from step 2 is incorrect.

Many people will just go with step 2, and that is just about the worst thing you can do, assuming that you actually do not guess correctly.

Good interviewers will not expect you to know everything about the topic, and they will know that the first step to improving yourself and your knowledge is being able to admit when you do not know the answer. This is actually a really important skill!
But once you have shown that you do not know the answer, showing how you would go about finding the answer is also very useful. An employer will not want an employee who just sits there and says “I don’t know” and does nothing about it.