Headshots can be a challenge. They are not merely a portrait as they rely on the person and not the environment to make the image. A portrait often takes liberties in presenting a person as the want to be seen, whereas a headshot needs to portray the person as they actually are. Nothing is worse for a performer than not looking like the headshot that got them the call in the first place.
Hence the reason why headshot are often frequently updated. If the performer changes their look they will need return to the photographer for an updated headshot. The photograph used for the headshot shows the performers age, style, look, etc. Any major changes in these will require an update.
Headshots are currently shot very tight. Seldom do you see a head and shoulders headshot anymore. Crop is typically 8×10 just below the chin and into the top of the head. The performer often faces slightly off to one side, but not in profile. A straight on shot is also used for headshots, much more than in portraits.
Both high key and low key are used for headshots. The low key shots tend to be the serious, non-smiling shots and the high key shots tend to be the warm, smiling shots. Models often go for a more glamorized photograph for their headshots, more along the lines of a beauty shot.
The goal of the headshot photographer to produce the series of headshots the performer needs. The minimum that a performer will need is a smiling and not smiling shot. The photographer needs to make sure that the eyes in the shot are alive and bring you into the shot, smiling or serious.
A great headshot does not take a fancy set-up. Some of the best headshot photographers work with a single light source and a reflector or two. The key is knowing how to use those simple tools create the look that gets them noticed.
A headshot is often seen by a casting director for a few seconds before in moves to pile A or pile B. Great headshots will get into pile A much more often than a poor one. The rest is up to the performer.