First off, you have to think strategically.
If your a genius, a crappy interview isn't going to destroy you.
But if your grades are mediocre than the interview will be the deal breaker. Keep in mind that the questions you ask should be segways to talking about your strengths, and why you are the best candidate.
I really can't stress this enough: TAKE CONTROL.
Keep in mind time restrictions,
You need to come up with a game plan and implement it according to the time allotted, your interview tactics for 20 min, will be different than an interview that is for an hour. A benefit for longer interviews is that they have more of a chance to access your character.
Thus, the questions you pose will be CRITICAL.
But don't worry, I find that you'll come up with questions as they are talking to you.