Recently I was reading an article addressing the importance of interviewees to always be conscious of their body language during interviews; I immediately thought to myself, "Excellent advice all around!" Body language is a primary form of communication, and makes up some 60% of those non-verbal ques and 'tells' we generate whenever we engage in any contact with others of the same species. I have noticed when dealing with some clientele, especially those who are accustomed to working for long periods in the digital world, that we tend to take things more at the surface value, and often miss some of the more, ahem, 'subtle' interactions which occur in social and professional settings. This may be partly from the removal of the referenced body language in daily interactions, or it may simply be that some have 'adapted' to communicate more with acronyms, protocol statements and snippets of code ... the problem is this may well be leaving a gaping hole in your interviewing skills.
"When we speak (or listen), our attention is focused on words rather than body language. But our judgment includes both. An audience is simultaneously processing both verbal and nonverbal cues. Body movements are not usually positive or negative in and of themselves; rather, the situation and the message will determine the appraisal." (Givens, 2000, p. 4) http://en.wikipedia.org/wiki/Nonverbal_communication
For a potentially more positive interview experience, realize that not only is your body language important during the interview, but also that of your interviewer. Learn to look for the signs of disinterest. Notice any change in position of the individual.
I can not tell you how many times diligently paying attention to those non-verbal cues, ticks and micro-expressions has allowed me to create a win-win in what had looked like a losing situation. Conversely, there have been times my lack of acknowledgement of these cues has created the need to exert much more effort than if I had just paid attention to my own innate knowledge.
Some good rules of thumb I have learned to love are:
First, pay attention to how the interviewer is positioned when you first meet them. Did they get up and shake your hand? Did they just point you to a chair? These can indicate the approach of the interviewer through the rest of the process.
Second, if the interviewer shows actions of disinterest, note it and change the tone. Do not take offense to an interviewer looking at their screen, or typing something out to a colleague. Note that they do not seem interested in the line of conversation. Perhaps what you are saying they have already heard 10 times before by other candidates. Notice this cue, look to change the dialog to something of greater interest in your experience. Think of something you have done that would make you stand out from the other potential candidates.
Finally, keep in mind that in larger companies each interviewer will be questioning you on different areas of soft and hard skills. This may also impact the type of body language seen from the interviewer. Their own comfort level and experience with the subject may bleed some uneasiness into the interview. Pay attention, look for the opportunity to help them lead the conversation in a positive direction and calm their possible apprehension.
Take the time to practice reading the body language of others - your spouse is always a prime example to pick on for this! If not, there is always those weird people in the park ... at the game ... or you could always just practice that great American past-time and play some poker with your friends! Just don't blame me when you lose all your money!
For some further study, take a look at this great video tutorial from eHow.com!