How to hire the right photographer
It's actually quite simple to choose. The most important thing is that you are comfortable with the photographer. You want someone with experience. You want someone you get along with and will be able to communicate freely with. You will want to see their work and like it. You want to be able to trust them based off their experience and the care that they put into knowing what you are going for. You have to trust them in being able to capture you and what your casting is. This will be based off of their ability to communicate with you. You can tell by the conversation you have and by looking at their work or website.
A good side note on this is that photography is an interesting art. It is both an art and a science. There are some photographers out there that are very savvy on the technical side, but they are not good at communicating with their clients and the pictures end up dry or lacking. There are also some photographers who are great at chatting, but lack the technical skills or experience to really get great shots. It's just something to be aware of when you are selecting who you want to do photos with. This is why you want to see their work and spend a little time talking to them.
A photographer with poor communication skills will have their clients looking very stiff and posed very formally most of the time. Or they will get in really tight on the shoulders and head in a studio with great lighting, but empty looks on their clients faces.
A photographer with good communication and very little experience will often have shots that look a bit like snap shots. One way to tell is the backgrounds. Beginners rarely see their backgrounds as important. They will be weird or out of place. They will seem boring. Also beginners often have washed out colors. These are just a few indicators of a green photographer.
In a nutshell you should like the work you see and feel comfortable with your photographer. Those two things are the most important.