When you’re asked a question about your research, unless you’re completely surprised or stumped, the shape of an answer appears in your mind. Think of it as a sketch of a response. The outline of what you could say. It’s suggested by your brain because of your experience and your talent.
Your first thought has given you a shape; a second thought can help you refine it a little more. If the shape is five reasons why such-and-such is a good idea, a second thought can help you quickly reflect on a good sequence for those five reasons. Maybe one is better than all of the others, maybe another isn’t as important as you thought at first glance.
In most cases in the viva, when you’re asked a question the shape of an answer will appear. Still, think twice. And if you go blank, if your first thought is “Huh?” or “…” or “I don’t know,” then have a second thought. Don’t let the pressure of that moment stop you from responding.
There’s plenty of time to think twice in the viva.