48 Hour Rules prior to the interview
When discussing the job and scheduling the interview, don't reach out to anyone but the person listed as the contact person. Some companies are strict about who talks to potential candidates. E-mail or call that person as your contact, and leave your supervisor or co-employees out of it.
Do arrive promptly, but not too soon. If an interview is scheduled for 2:00 pm, getting to the site at 1:30 pm doesn't spell enthusiasm, it shows a lack of understanding about time management. Plan to arrive about 10 minutes before the set up. If you can, make sure you know exactly where you'll meet ahead of time.
If walking around the site, don't point out flaws in either buildings or procedures. There's nothing worse than being the candidate who points out that the corner of the building has a major crack in it or the one who asks, "should they really be violating OSHA regulations that way?" You will have plenty of time to express your concerns later at home, when you're deciding whether or not you want to work there.