In interviews and vivas, the people who do best are not always the most qualified. They are the ones who can communicate what they know clearly and calmly under pressure.
That is a skill, and like any skill, it can be prepared.
Before the day
Know your story. Prepare a two-minute version of who you are, what you have done and where you are going. Use the Story Spine if it helps. This answers "tell us about yourself" and gives you an anchor for everything else.
Prepare three examples. Choose three experiences that show your strengths: a challenge you overcame, a project you led, a mistake you learned from. Most questions can be answered with one of them.
Practise out loud. Thinking an answer and saying it are different skills. Practise with a friend, or record yourself.
In the room
Pause before answering. A two-second pause looks thoughtful, not unprepared. It also gives you time to organise your answer.
Answer the question first. Then explain. Panels lose patience with long build-ups.
It is fine to say "I don't know". For a viva especially, saying "I haven't looked at that specifically, but my thinking would be..." shows honesty and reasoning. Bluffing is usually obvious.
Treat it as a conversation. The panel is not your enemy. Most interviewers want you to do well. Speak to them as people.
Afterwards
Write down what went well and what you would improve, while it is fresh. Every interview, successful or not, is practice for the next.
Preparation does not remove nerves. It gives your nerves something solid to stand on.