You make an impression the moment you walk in a room. Do you stand erect? Do you look people in the eye? Do you engage with a smile? These nonverbal cues speak volumes before the questions even begin.
Posture implies qualities such as confidence and leadership. When sitting, don't slouch in the chair. Sit erect with hands placed comfortably in your lap. You will be nervous so try to be conscious of fidgeting. Holding something such as extra resumes or a folder may help.
When answering questions look the interviewer in the eye. Numerous studies show looking in certain directions demonstrates your subconscious thought process. For example, looking up towards your brain is common when retrieving technical information to answer a question. Looking down towards your heart implies a more emotional response. While you can't control these involuntary reactions, staring at the ceiling for an extended period will make interviewers wonder if you are trying to think of a better answer than the truth. Looking them in the eye builds trust.
The importance of a smile cannot be underestimated. You have been given the opportunity to interview so you should have plenty to smile about! The interviewers likely have a long day and they may be tired but engaging them with a smile triggers an emotional response. Most likely they will smile back and that simple act makes them feel good. You want the interviewers to feel as good as possible while they interact with you! A smile also adds a sense of levity to the room and helps everyone around the table relax giving you an opportunity to shine.
It is difficult to be consciously aware of these nonverbal cues in stressful situations so keep them in mind and practice with friends or family until it becomes routine.