Style: you must be familiar with the photographer's work to see if it meets your expectations. Some are more forward and direct and organize photos. Some have very traditional formulation, while others use high tech cameras and exciting prospects. You can get some flexibility from the photographer, but do not expect a photographer whose style does not fit your needs to change drastically.
Personality: you will spend a lot of time with this person, so you have to be very comfortable with him/her. Make sure your family and the photographer will be compatible.
References: If you've heard about them through a friend, they come highly regarded. If you find them through the search engine, ask to see examples of the images and talk to the bride or other persons to whom they have worked with earlier.
Pricing: You may want to set a couple of scenarios and ask the photographers to price them according to their abilities. Be careful here as well: the cheapest photographer may not be the best. You want to find a combination of value, quality, and range of products.
Product choice: If you are mainly interested in digital prints, it is highly recommended to choose a photographer who can produce good large prints and albums because of their experience in making these images show, they have skills and resources to produce good products, whether they are electronic or printed.
Digital photography: Many photographers offer this service. Ask whether the images will be edited or raw, and the extent to which editing will be performed. Any size or resolution will be proposed. Or releases shall allow you to print in a laboratory? Are there any restrictions to your use of images? Most photographers will retain copyright and ownership and provide you with a personal use license.