I am not sure if there are any people who can advise on the specifics of the interview (hopefully someone can though).
However, if it were me, I would be polite and professional, but I would say as little as possible and just answer 'yes' or 'no' to their questions.
I have a large issue with an issuing authority utilizing someone who is trained in interrogations treating a law abiding citizen like a criminal and trying to dig for a reason to deny them their human right to self defense.
You will likely get alternate advice here (perhaps for good reason), but I have learned from enough police encounters the bare minimum is necessary and the only thing you can give them by talking is more ammunition against yourself. That includes if you have nothing to hide.
One thing I will ask is if you are able to record the interview (no consent from the officer is needed), that would be helpful for everyone to know what they put people through. At the very least, if you can try and write their questions down while they are still fresh, that could be interesting.
Remember, it is their obligation to perform an investigation to find a reason to deny you, not approve you. They will spin it the other way around if they get the chance, but lacking any question of suitability or any criminal history in the FBI report, they have to issue it to you. If you are the clean person most people who apply are, you only have something to lose by giving them information.