Interview after principal approval

I applied for my immigration under family sponsorship category after I got married here. When i call the call center they say that my process has been done and I received a letter today that says that I have been scheduled for an interview in June. They asked me to bring 2 photos, and $490 as the fee for permanent residence. They also advised me to bring my wife with me. I am just confused if its a interview or they just want to meet me before conforming my permanent residence status. Because if it was an interview they would have told me to bring more proofs or anything. I am so confused because if it is an interview me and wife have no idea what kind of question they are going to ask or how long that interview will go. If any of you have been through the same situation please help us to get an idea as what is going to there. 

You are likely to be asked a number of questions related to confirming the information they already have processed, and related to current circumstances, most notably of course that you and your sponsor continue to be in an exclusive, committed partner relationship and that you do in fact live together (a condition of inland partner sponsorship).

As PMM says, they are probably satisfied as to the bona fides of the application, so no additional proof is necessary beyond you and your partner confirming the specific elements of eligibility to become a PR via the inland sponsored partner process.

Even spousal sponsored PR applicants in the outland process are "interviewed" as to certain essential elements before they land and become PRs, even though the visa has already been issued. While such interviews are largely a formality, perfunctory in nature, they are a required formality and they are essentially the last check stage in the process, inquiring in particular as to whether there has been any material change in marital status, criminal history, and such. Similarly for those in the inland process except for the additional requirement that the sponsoring partner as well as the applicant confirm the continuing nature of the relationship and continued cohabitation. Pretty simple and straightforward, and for legitimate couples actually living together, very easy, pretty much stress free.