The most important thing to remember is that you don't need to get it right the first time you press record. You can record and re-record as many times as it takes till you're happy with the video.
Take a deep breath before you begin filming. Talk slowly, don't try to rush through your response.
If you mess up, don't panic. Just try again and remember to speak slowly and clearly (and remember to breathe!). Your director won't let you look bad. You just need to speak from the heart and the rest will fall into place.