We just went for an interview last month and my partner was approved and got his coPR from the visa office 2 days later. We had a LOT of red flags and we knew there was a high probability that we would be requested to go for an interview. My advice would be to order GCMS notes to see if you can find out what are the reasons for the interview, so your wife can prepare better to answer their questions. (If your interview is more than a month away, you should be able to get the notes before the interview if you order today/tomorrow). Often it will state their concerns when they summarize your relationship. For us, it was not stated, but we already knew the main reasons they had doubts about our relationship. Sometimes it can be very simple things they want to clarify or major concerns they have, like in our case.
My feeling after going through the interview is that if you answer their questions confidently and with conviction, even if they seemed to have made up their minds to refuse the applicant, it can go a long way to change their minds. The interviewer told my partner that before she talked to him, she was really leaning towards refusing his application, but talking to us in person made a difference i.e. she could see we had real intentions of being together. I, the sponsor, was not asked to go to the interview, but I did go and the visa officer ended up talking to me also. I think in our case it helped and for some other applicants I have read about too.
As for the passport, he actually needed it to only get into the embassy. Now they did not approve him immediately after the interview but the visa officer told us she thought our relationship was genuine but she still had to check his documents he brought in (new PCC and driving record from his country and also his medical exam results). The next day they did request that he bring in his passport to the embassy and then the following day he brought it in and 2 hours later he was issued his coPR. It is not a good or bad thing if they request your wife to bring her passport ... it might be just for ID purposes or if he passes the interview, they will often ask that he submits it there to process the coPR.
Good luck with your interview! Hopefully you will feel more confident if you are able to anticipate their concerns and prepare for it appropriately.
