In the event that a photographer has to cancel at the last minute always ask if he or she has someone that can fill in and be sure to talk to that person and see a sample of their work. If you don't like the potential replacement, consider having a friend or family member that could stand in or even talk to a larger company that has a greater number of staff photographers.
Talk with your photographer about all of those important shots that you want. You will want a wedding photo of the groom waiting for the bride, the flower girl, the bride and her father walking down the aisle and of course of the couple after they are married. What others are important to your family?