Congrats on your interview date, that's great news!
As for advice for the interview....
Casa has been known to be a difficult consulate, so it's best to be prepared. My husband's interview was very easy, as his interviewer was one of the "easy" ones, a young man. There is a woman who tends to give beneficiaries a hard time, particularly if the relationship involves a large age difference (younger man/older woman). Even if there are no such "red flags" in your case, I'd recommend that you push your evidence on her, even if she doesn't ask for it. Make sure that your fiance brings lots of proof of your relationship to show, and that he's prepared to talk about it.
Best of luck to you!