Bruce Herbelin- Earle 2015
Until recently an actor was often selected on the basis of his/her headshot. Going back actors headshots tended to be B & W today they are colour in the main in the UK.
The big mistake is trying to see a headshot as something it is not intended to be - the best picture of you fully retouched and made up. That simply is not the point.
A headshot should be the actor walking through the casting office door. If you have teeth with braces - show them - if you wear prescription glasses include shots with and without where practical.
There is a trend to go for almost "out of focus - soft focus" headshots. Not normally a good idea.
In fact if you are agent represented you probably will have strict criteria or be shot by their usual photographer to keep all the actors equal in presentation. It is vital for growing actors to keep headshots updated, and be sure what you show is how you look .
The exception to this might be if you are in a role which demands say short hair when your headshots have a longer cut. In these cases declare this and send if you can an image updated.
In recent times the headshot is being pushed back a bit by the demand for self tapes. These are shot for a casting normally at home and often by an i phone or some similar device.
Most castings will provide instructions on how to present - be sure to follow them.