Talk to some cops for suggested questions to work with. Research and read about oral interview questions for applicants. There is a science to it - or as I say, a method to the madness.
Once you find some questions, practice them in front of someone and make sure to do it aloud. Sitting in your room whispering them will not cut it. Standing or sitting in front of a mirror may help but you need to get used to the human interaction...and follow-up questions. Again, practice these aloud; your mind needs to get the words out of your mouth, not a rewind in the brain. Make it natural.
Integrity questions are common place.
The usual line of questioning includes a scenario such as, "You find a store open and there is a senior officer helping himself to some goods. Now what do you do?" Do not laugh or say, "Something like that would never happen with this great department!" The interviewers do not want to get a warm fuzzy feeling; they want real answers.
The standard answer of "I would follow the book" is a train wreck. We are talking about integrity - and yours, at that. A department is no better than its worst employee. You will have to validate your answers. And just when you think that question is over, there will be follow-ups; be prepared.