I'm just curious, What exactly is the interview for? what usually is said / discussed at the interview?

I think they implant a tracking device and then interrogate you to learn all of your secrets. Then they give you a pill that makes you forget everything they did and then they pretend to ask you a few simple questions and act like nothing happened.

When I did mine in the early 80's it was all American Civics.

Like if the President and Vice President dies who is next in line and what's the name of that person

Aside from the Civics and enligh profficiency test, they go through your application. If you are there by marriage, they go over all your proof of marriage was in good intentions, they look to see how strong the marriage is. They will go over legal matters, if you've been arrested, been to court for anything and other moral things. They'll ask about trips you've taken since you've been a PR, they'll ask about your tax filings and in some cases ask to see the tax reciepts.

So quite a bit of things they'll go over, basically it's like a job interview, you're trying to show them that you're qualified for the position of being a USC and meet their job requirements...