Interview is usually to check marriage genuineness. The red flags are typically:
Different social status/culture/origin/religion/country,
previous relationships or divorce on either side,
lack of compatibility,
significant age gap especially on the wife side,
short per-marriage courtship especially if your marriage is not family-arranged,
lack of marriage publicity (e.g. did not do marriage ceremony/wedding or your marriage ceremony/wedding was not attended by your close family members),
sparse photos.
lack of continuation of contact
lack of financial interdependence between the couple
Hastened marriage
lack of cohabitation or marriage consummation.
If you have any of the above concerns, you should have written a detailed narrative statement addressing all these concerns and enclose it with the application so that the immigration officer understands the reasons of these things. By doing so, I was able to waive the interview. The ground of refusal in interview is usually based on LACK OF KNOWLEDGE OF THE PERSON BEING SPONSORED WITH THE SPONSOR. Therefore, prepare for the interview very well by knowing all the details about your sponsor.