When you're seated to begin the interview you will want to be as comfortable as possible. At the same time you will want to appear as enthusiastic and professional as is appropriate under the circumstances. At all times you should sit up straight and towards the front edge of your seat so as to appear eager. Do not lean back in your seat and definitely do not slouch in it.
To prevent your hands distracting others, simply cup them together and let them rest either in your lap or on the table before you. This should prevent you from making any gestures that can detract from your attempt at a professional image. If you do find yourself having to use hand gestures, then the smaller they are the better. Keep all your gestures subdued. If you can't hold your hands together, then perhaps hold onto the pen that you brought along. Just don't be tempted to tap with that pen because that is worse than distracting - it is irritating.
Some other things that you shouldn't do are: chew gum, twiddle your thumbs, hum or whistle a tune, stretch any part of your body, find yourself staring at someone, use slang or swear, fidget and touch anything that isn't yours that is on the table or in the room. Never cross your arms because, not only is this a very defensive position, it also creates a very negative impression.
When you are asked a question - even a difficult one - do not cast your eyes downward, or look up towards the walls and ceiling for the answers. You won't find them there and it is somewhat childish. Instead strive to maintain eye contact with at least the interviewer who asked you the question. Once you've completed your answer look to the other interviewers and smile politely. If they are pleased with your answer or with how the interview is going, you will find that most interviewers cannot help but smile back at you. If they don't smile back, then it is a signal that you still have some convincing and selling of yourself to do.
We all know that a picture is the equivalent of a thousand words and in an interview situation your face can be worth several times more. Your facial expressions will convey your sincerity and honesty in answering the question. They can add as well as detract from your words, so be careful not to wince or pull any negative facial expressions. Making a concerted effort to end your answering of especially difficult questions with a slight, but natural smile should overcome your giving away feelings of discomfort or any other negative emotion.