Listen carefully to the questions you're asked, and try to relax by breathing calmly from the diaphragm. Although you may think they are, some interviewers are not as calm and comfortable as you might think. Sometimes they've not prepared adequately for the interview, and their questions may seem odd or irrelevant.
If you are asked an open question (usually starting with What, Why, When, Where, Who and How), you have the opportunity to give a full answer. A less helpful question is "Tell me about yourself", which gives you huge scope, but often leaves you feeling you've given an incomplete answer.
Closed questions require you to give a simple no or yes in reply, for example, "Have you ever worked in the private sector?" However, the interviewer may not have formulated the question properly, intending you to give a much fuller answer. Protect your interests by making sure you understand the question (it's sometimes useful to repeat it back to check), and then answer as accurately and precisely as you can.
If you don't understand the question or wonder if you have answered the question correctly, check with the interviewer. You could say, "When you ask x, do you mean ...?" or "Have I answered your question, or would you like me to go into more detail?"