Some good info on here guys, some bad advise also. If someone told me the interview was over, for whatever reason, it would be.
I am a Service Manager for the company I work for. I have been in the field for 14 years. I would say "HONESTY is the best policy." When I interview someone, appearance, willingness to work, and confidence are some of the first things I look for.
If you go to an interview and your clothes are wrinkled, hair a mess, haven't shaved, earrings on, tattoos blaring, odds are your not going to have a chance. In the same hand, a suite and tie would be over-kill.(yes earrings and tattoos are a downer at first glance. Give the interviewer a chance to get to know you before you hit them with that. I wear earrings and have tattoos down both arms but when meeting a possible employer for the first time, I covered my tats and took out my earrings. Ask that question sometime during the interview, most residential employers will not like the tattoos, but if you are honest about it and ask if tats and earrings are acceptable, you will be told what they want to see and don't want to see.)
Let the interviewer know that you're ready to go to work, have the tools necessary to perform the work, and that learning is still job one. Ask any of the guys on this site and they will say that they don't know it all, and still have to ask for help on occasion, I know I do. The interviewer is a person too, they were probably in your shoes at one point. Treat them as you would an elder, good manners and such, and that will give the respect due.
Honesty, Honesty, Honesty, if they ask specific questions, like; Can you replace a burnt out compressor by your self? or Can you work on VAV systems? or NOVAR? or EMS systems? TELL THE TRUTH!!! If you don't it will be obvious soon enough, and could lead to a pay decrease or termination. If you have experience with the stuff tell them what you do know, let them know that you are not afraid to ask for help. My rule of thumb(for myself) is, if I havent figured it out within about a half an hour, call someone.
Also....don't be afraid to ask questions, about anything. Make a list of things you would like to know. i.e. What are the chances for advancement? How long has the company been in business? How often are reviews? Are there paid sick days? What holidays are paid? What kind of bennies do they offer?
Hope this helps.