The best way to answer questions calmly during an interview is to take your time. Reflect the question back to the interviewer to make sure you understood it and to give you some extra time to compose your reply: "A time I overcame a challenge at work was..." Speak slowly. And don't be afraid to say something like, "Let me think about that for a moment" before answering. It also helps to prepare some answers to commonly asked interview questions in advance. While you don't want to memorize an answer word-for-word, having some examples in mind will help you feel more relaxed when you hear the question.