Unfortunately, a relationship interview is always a bad thing. I know in a previous thread you were really happy about getting an interview because you felt the process was moving forward, but in reality, its a roadblock because it means the VO has concerns your relationship is not genuine.
Even if you have lots of proof it is genuine, the fact is, we've heard many stories of interviewers dismissing people's evidence and making up their minds from the get-go, or people just simply getting nervous and giving incorrect answers.
I've had a friend of 5 years who tried to sponsor his Viet wife, they even had a kid together prior to submission, and I know they're as genuine as can be, but after the interview (they got an interview because he initially withdrew due to arguing w his wife, but reconciled), they got refused.
You could be the most genuine couple and still get refused because the interviewer might have a bad day or doesnt like the way you compose yourself. So now your best course of action is to determine what your red flags are and do the best to address them at the interview