i'll have to disagree that the ONLY reason they interview is in question of the relationship... i've heard of interviews where they just want clarification on an aspect of the application they found confusing (heard of someone who all the interviewer asked them was to describe in more detail how they met (was online)), have also heard that some for cases (i.e. most ARC cases) it is very common to require an interview even if they have accepted the relationship... read of a case where the interviewer just asked a few questions confirming the application (very easy questions, date they met ect) and then told the applicant they went thru the case and believed in the relationship and was recommending it for approval and asked them to pay the ARC fees (they were told to bring in the payment, so i guess the interview was just a technicality). in fact it's not uncommon for people to come back from their interview and say that the interviewer was very nice, asked a few questions and told them they were recommended for approval.
i believe a certain percentage of interviews are just "quality control" if you will... if it's called very early after approval, maybe it falls into that bucket... I think there would have to be blatant holes in the application for them to have quickly decided that the relationship seemed not genuine without an review..., so, if you feel your application is solid and have included good proof, i wouldn't worry... (usually they will ask for further proof if they question it, won't they? i've seen this plenty, they ask for more documentation and give examples of the type of things they are looking for)
just my rambling thoughts... i'd say be confident and answer honestly and bring everything you can to support the relationship just in case.
good luck!