If you were going into meet a casting director or agent for the first time, what would you tell them about yourself... what roles would you see yourself playing and what is your personality like?
Think about the differences between acting for stage and acting for film. Although most actor headshots are the same for either, it does help to decide what kind of look you are going for. Do you play more serious roles on film and more comical roles on stage?
You should have different looks for different submissions and you should opt to get as many looks in as possible during your headshot session. If you don't plan on using your shots for many roles, then you might consider doing a much shorter headshot session to be able to just focus on one role while saving money. Typically though, it's best to do a longer session and get lots of looks and roles in.
So how exactly do you decide which roles to play? Look up shows and see different plays and movies. Be familiar with the characters. If you know a character you would love to play, why don't you write it down? Keep a decent list to show to photographers when you are doing your headshot session. They will much more easily get what you are looking for if you give them an example.
Relate to a character you know and think about character types. What characters and people in your life do you know and would they would be a good fit for you? Can you play them? If so, you should definitely consider using them as models for looks to go for in your headshots. Think about them and the expressions they make as you take your headshots.
What's your sense of humor like? Everyone has some sense of humor. And actually some sense of every expression. Some people may show expressions more than others, but we all have it, and we all have different types of each as well. By this I mean that you may actually be a sarcastic funny person. Or you may be a silly funny person. There are many different types of "funny" you can be, just as there are many different types of angry or dark that you can be. Think about what you'd like to portray before the headshot session.
Lastly, ask what kind of person you are. To take good general headshots of yourself, you want to bring out your own personality. A headshot photographer should be able to do this, but if they cannot, it is okay because you should be able to. If you know your own personality, just try to be yourself and show it off for some of the shots. It's important to have headshots that show your true personality in addition to all the looks you are going for.