Experience - How much experience does this person have with your type of event? Is this their hobby or their job? For example, I do photography for fun because it's something I love. I've been doing it for a while and still have a lot to learn, but I can certainly give you a good product as well. Which leads me to my next question: do they have a portfolio (or blog) you can see?
Their Style - What style are you looking for when it comes to the finished product? Are you looking for documentary, formal, whimsical, classic? Study different photographers (if you do not know them, I'll research and send you some to consider) to get an idea of what you want. Something in particular to think about is posing. Some photographers are really good at still shots, but not so much with candid photos. What is most important to you? Think about it and look for those characteristics in their photos.
YOUR Style - More importantly, what is your style. Look at your past photographs you love of yourself and others. If you're not comfortable with posing or "modeling" don't hire someone or let someone pose you. You're paying a lot of money, you should get what you want.
Appearance - Meet with different photographers to get a feel of how they present themselves and their staff. If they show up to a meeting in their pajamas, is that really who you want? How do their staff dress and present themselves? Staff may be new to the business but they should still know how to dress. A photographer (and videographer) should dress for the occasion and blend in, not stand out.
Personality - Along with appearance and your style consider their personality. Can you talk to your photographer? Does he/she understand what you want? Are they willing to work with you on this type of event? Is their a "chemistry" between you and the photographer or is it completely awkward?
Client References - Ask photographers for references you can talk with, not just listed on the website or blog. Get an idea of what others thought and who they would recommend for your type of event.
Vendor references - How well does the photographer work with other vendors? Talk with your videographer, caterer, venue and see if they worked with the photographer before. Who do they work well with? How did their experience go with your potential photographers?
Other items to think about:
- pricing
- proofs
- turnaround time
- contract and requirements/limitations
- equipment
- time available
I know this is a lot to think about when choosing a photographer, but considering all of this will go a LONG way in making the actual event run smoother and ensure you get the product you want.