As 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 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 tell a persons personality just by looking at their face.
If someone looks very friendly, they usually are. If they look mean, they probably don't have the friendliest personality, because over time, their expressions have caused wrinkles in their face in that way.