I've interviewed many people and most people are terrible in that situation, even when I try to help them along.

Nervousness is the number one thing that throws people off during an interview. Instead of taking a breath to calm down and think things through they either immediately start speaking without a clear plan, or they clam up and don't give me anything to talk about. Using the STAR/START method of answering questions works great to give a framework.

The next is that they just don't demonstrate their skills or that they learn over time. I'll ask a technical question about something and they show no understanding of why they did something. I love asking questions about what a person learned while doing their work, so many people just don't have anything to share.

Lastly, interviews are way easier if the other person treats it like a conversation rather than a one way Q&A session. I get a much better idea of what they will be like to work with and I'm more comfortable when I can ask questions that continue on the discussion. On the flipside, some people take over conversations and don't get that our time is limited and answers need to be concise. That said, even if the interview is the conversation style, make sure to save at least a couple questions for the ending when you are inevitably asked for questions.