As someone that conducts these interviews, I think I can help with this answer a little.
First and foremost, be honest. Let your interviewer know your concerns and desires, they may know some ways to help you succeed.
If it's in person, be on time and dress appropriately. The one thing that has rubbed me the wrong way the most was when I was to conduct and interview and the person didn't show up on time or seemed very disinterested. Remember, the people doing these interviews may have more than one to complete, have pts to see, and have other duties that they need to see to. Being late or showing up in a t-shirt sets the wrong first impression. Essentially, this is a job interview and you should really treat it like one.
Make sure you have all the requested information sent to the interviewer before the interview and try to make sure it gets there so they have plenty of time to review it before the interview.