Before you ask anything, you need to love their work and "click" with your photographer. They will be with you more than anyone else on your day so it is important to get one well with them.
* Do they back up their images
* What do you need to do to get a copy if you lose yours
* What "style" do they use - is it a set style or varied and what do you want
* Do they accept photo suggestions - if that is something you would like to do
* Is there a contract and what does it cover
* Are they insured
* Can you see a full wedding portfolio - not just what is displayed online
* How long till you recieve your images and what is the guarantee offered
* How many images will you receive
* Have they done many weddings
* Will the photographer you meet be the one on your day
* Ask to see past clients testamonials
* Has there been any cases when the Photographer did not attend
* Do they get on well with other vendors - especially the videographer as they often need to work around each other
* What will they be wearing - you don't really want large logos being advertised at your wedding
* Ask to see the quality of their products - albums - acrylics - if a part of your package
* What other services may they be able to provide
There are bound to be many things to consider but again the most important is do you like their work and if you get on well, you are at least half way there.
Happy planning!