The first thing you need to know is to start your research and look for a couple of photographers that you can work with. Don't jump the gun and go with the first photographer you find. At the end of the day, you will want to go with a photographer that has experience in your field.
For example, if you are looking for acting headshots, you should find photographers who specialize in these kinds of photos and who have worked with actors to create their profiles in the past. You'll find that some headshot photographers even have a list of clients who've created success for themselves and used their services for their headshots. There are many online resources where you can explore more about professional headshot photographers.
Next, take a look at their portfolio to evaluate their work and see if they have produced great shots for their past clients. Some photographers will only provide you with some samples but you will eventually want to go with a photographer that has a huge portfolio, with varied clients, that you can look over.
Finally, let the photographer know why they are taking the pictures. If you're looking for acting headshots, then let them know as they will understand what kind of photos you want and which don't. In conclusion, you should take some time to evaluate several headshot photographers before you hire their services. In some cases, it will be worth it to pay the additional services fee to get a photographer that can do the job right the first time. Remember that the right headshot can make a big impact on the advancement of your career.