Follow the leader. When put on the spot, listen and gauge what the other person wants from you. Whether it's specific facts or simple reassurance that you're on top of your game, assess the situation before giving too much away.
Ask questions. Make them feel more engaged, while using the opportunity to glean information and trigger knowledge. Keep them talking - 70:30 in their favour is a good rule of chat.
Stop the negative voices. Concentrate on what you do know, rather than worry about what you don't. Replace 'fear of failure' with 'chance to learn', and embrace the opportunity.