Take a moment to think about the purpose of the interview. In many jurisdictions, there are more (or far more) qualified candidates than there are nom slots. The committee is trying to decide whom to nominate. They have the paper you submitted. Now they want to learn more about you as a person. So, you want to make yourself standout and be memorable, in a good way. Think about why YOU they should nominate you over the many other well qualified candidates who "want it really badly."
First, be very familiar with the package you submitted. Reread any essays you were asked to write. Review your "resume" or similar document. If you've been asked to select one SA or to rank SAs, be prepared to discuss your decision. Display a good understanding of what the SA and the military are about. Think about what makes you a great candidate -- and walk the fine line between confidence and braggadocio.
Also, there likely will be a few "off the wall" questions. There is zero way to predict them. Some examples are "What are three adjectives that best describe you?" "What is the last book you read and why did you choose it?" "Who is your hero and why?" And on and on. As noted, there is no way to predict those questions because they are up to the individual interviewers. Just take a deep breath and answer to the best of your ability.
Finally, never disparage another service. Don't even joke about it. You never know the backgrounds of those on the interview panel and "dissing" another service is generally frowned on by all who have served.