Confidence is key
Be calm. They want you to succeed. They've spent a bunch of money trying to find you. They want to get the interview process over with and fill the position.
You've prepared for this interview your entire life, read a bunch of books on your subject and burned many hours working out exactly the kinds of problems they need solved. Sure there's more to learn, but you know the lingo and have a fair knowledge of the landscape. There will be more to learn, but if they expected you to know buckets more, they'd be offering a bigger title and a lot more money.
Relax.
You have to understand what every company wants: an employee that will show up on time, put in an honest day's work and get along with everyone else. They hope for someone who is willing to learn and put in extra effort when necessary.
Knowledge of the company would be helpful because a typical question will be, "Why do you want to work here?" or "What do you think you will add to the company."
"Well, sir, ABC company is a leader in the industry," or "ABC works in the area of intergalactic widgets, a special interest of mine. Your model 123xyz is cutting edge and I enjoy a challenge." (You won't just put in the minimum time and effort.)
Proving you've done your homework would be a real advantage. "I spoke to several employees, including Foreman Jones. You treat your employees well and reward hard work. I'm sure I will have a long and bright future here. (They want to know you will stay a long time and work hard.)
What you don't want to do is sink your own ship. One popular question is, "What do you think your biggest problem is," or some such. Only a fool will say things like, "I can't get along with authority," or "I steal stuff."
The better answer is one that really isn't a problem for the company. As a graphic artist and copy writer I used to say, "I have trouble going home. If it wasn't for deadlines, I'd continue to polish an assignment forever." (Extra effort and a drive to do good work.)
I had one interviewer ask me, "If you were an animal, which would it be?" I told him I'd be a tiger. "Sure," I said, "everyone wants to be strong, ferocious and beautiful, but tigers are good parents and they mate for life."
Tigers don't mate for life, but any interviewer will over look this 'error' for the substance of the answer.
I don't have to tell you to dress appropriately do I? Clean and shiny, ready to work. Sit up straight, speak clearly and calmly, look your interviewer in the eye. If you don't know something, say so, and add that you know where to find the answer, or ask for a source you can track down later.
I did make one mistake I should mention. (Well, one big mistake, anyway.)
I was talking with another guy, a fellow employee from another department. I told him I had just read, "Dress for success" and I was going to go blow a pay check on new clothes. He told m that he had a better way.
"Don't dress better than your boss, man. Dress just like your boss."
I took his advice and bought an outfit just like my immediate supervisor. However, when I showed up in it, I was fired on the spot.
I think she was jealous.
G'luck.