To be a professional headshot photographer in New York City and Manhattan, you have to make sure you know the client's personality as soon as you meet them. Learn their mannerisms and see what they act like and do.
There is no easy way to learn a person's personality except by speaking with them during the session. Remember that if they seem not to joke too much, a very smiley and playful shot will probably not fit them well. It won't capture who they really are.
On top of that, you have to understand that it's not only just about expression. The lighting and style of the entire photograph should serve to capture the client's personality. There is something about every headshot that either makes it truthful or doesn't. Look at the shot and try to determine: does it look natural?
The funny thing is that you can actually tell a person's personality just by looking at their face. If they seem to have a mean face, they are probably not the nicest person. Wrinkles form on people's faces after the age of twenty-five that show the expressions they most often make, and you will know right away how amiable they probably are.
But on the other hand, you have to know that sometimes people take a few minutes to break in. Something may have happened that put them in a bad mood, and if they seem like that in the photos but are usually fun people, you might have a problem. Being a good headshot photographer, you have to know how to talk to people to bring out who they truly are in every day life.