There is not any requirement for just how secure a location you need as long as the firearms are secure, and not accessable to minors, prohibited persons, etc. I dont know the area you are in so I can not comment more as to security. As to in your house or garage, I would probably have a room on the garage with a seperate entry door. That is just my opinion.
The interview will be just a sit down with an inspector, going over the regulations, book keeping, if you make a mistake, how to go about transfers/sales, what is required, and what that inspector/office likes to see done, or more of what they look for to be done.
Keep in mind there is some variation between the field offices and each inspectors views of the laws/regs may vary as well, so feel free to ask questions and see how they feel about any scenario you may have in your head.