There are many books in your local library which will tell you about the scientific study of body language and nonverbal communication. Basically, we are all animals and respond to each other on a simple level in this way. When we meet other animals we need to know that we are not under threat. That is why smiling at another human being is such a powerful signal.
When we smile at other people we reassure them that we are not going to attack them, and being smiled at by others is the way we receive reassurance that the person facing us is not an enemy. Communication between people is much more relaxed and straightforward when we know we are safe.
Think about how difficult it is to make conversation with your dentist before you have treatment. So, to create a good impression, start off the interview on a positive note by entering the room and smiling at all the interviewers present. Even if you are too scared to smile again, you will have started the interview in a confident way. Shaking the hands of those who will be interviewing you is a helpful way to start if you feel confident in doing so. Obviously, if a cumbersome table blocks the way between you and a large number of interviewers, attempting to shake hands may cause more trouble than it will be worth, but if possible, a firm, friendly handshake makes you look open and positive. This applies particularly to women who can make an impact this way through rarity value, as shaking hands is still less common amongst women.
At the end of the interview when you leave, make sure you thank the interviewers for the time and attention you have been afforded and smile again to leave on the same positive note. If you began by shaking hands, leave the same way.