You may or may not be called for an interview after you submit this form. Procedures have been in flux for the past year or so regarding biometrics collections (fingerprints, and photo) and you may get a letter sending you to your DO or ASC for this service--this is not an interview. Simply follow the instructions that YOU receive from USCIS and don't worry too much about other people's situations. Yours is the one that counts for you. If you are selected for an interview, it will be at your local USCIS office, not at the service center. Most typically, you have already had a biometrics appointment before your notice of approval, and your new Green Card will be mailed to you. However, you should follow all directions received from USCIS. I-751 cases are generally completed in 6-12 months, currently

7.7) ..Is there another interview required to lift my conditional status?

A..All though the vast majority of jointly filed I-751s do not have an interview, you may indeed be required to attend an interview. Some files are selected for a sort of quality control check on the system, and may be transferred to the Local Office for interview. These cases may receive an RFE from the local office, and then find that the interview is cancelled. There is also a growing number of people being called for interviews (4/07) and there is a suspicion among the online community that PRs who did not have an AOS interview (via transfer to CSC or via the DORA program) may be automatically put in the interview queue for I-751.

At any rate, there should be no automatic assumption of a problem just because you get an interview letter.