Since the foreign fiance usually cannot come to the U.S. to visit the U.S. fiance, the U.S. fiance must go there to personally meet him or her. Documenting this meeting is critical. Photos prove they've met, but not when. That's the critical part and the reason for many rejections or delays. The INS is aware of this.
Insufficient evidence leads to questions that cause long delays and possible rejections.
Upon approval, the petition is sent to the U.S. Consulate in the region of the foreign fiance. Those are usually returned to the Consulate, wherein the Consulate schedules a visa interview. Just prior to the interview, the fiance must take a medical exam as directed by the Consulate. The visa interview is where fiance's either get good or bad news. If all goes well, the visa is granted within a week or less.