Do not be fearful to declare you don't know an answer.
Think of meaningful questions to express that you're curious and have thought about the company.
Try not to talk poorly about current team mates, directors or suppliers.
Check that latest social messages are professional.
Don't worry by thinking of pessemistic happenings.
Eat an energising meal beforehand - but no smelly meals!
Sit in the car, be calm, take some time to set yourself ahead of entering reception.
Suggestions for those looking for employment:
"The majority of jobseekers think they can't spend time from course work or their exhausting job. No matter what the excuse, the result is one and the same - any job search will decidedly stumble and you'll miss opportunities on significant employment rounds."
Key things for this position:
A colleague who doesn't mind to 'chip in' whatever the problem
Relevant background in a related job and over one years' involvement in this industry