Below is the redux version of my conversation with director Billy, he has directed shows at the Trafalgar Studios, The Print Room and The Riverside Studios, so he knows a thing or two. This is what he thinks about headshots...
It MUST look like you. I've been frustrated time and again by calling in people (certainly from things like CastingCallPro where I'm less likely to have seen them already) and finding they don't look anything like their head-shots. It's always worth noting that, for all the stress actors go through to prepare for auditions, the other side of the table is equally torturous at times. It can be long days of meeting person after person. By the time the end of the day comes and we start looking back over CVs and Headshots, we want to be able to connect actor to photo easily.
Similarly, I've seen photos of actors wearing broad smiles, or piercing intense eyes. If that's what your face naturally does, then fantastic but don't portray yourself in a photo as something you're not. I want the person who walks in to be the person who's photo I have in my hand. If the photo shows them smiling, I want them to be a smiley person. If the photo shows them as intense, that's why I've called them in. I want to see that intensity.
Having said that, do have a few possible headshots ready to go. Make sure they all fit the criteria above, but after that, see if you can get a slew of shots which express the (minor) variations in who you are. If I'm casting a period play, don't send that headshot with the shaved head. If I'm doing a comedy, don't send the frowning shot.
I want you to use your headshot to make my decision for me. I want to see it and see how that kind of actor fits the kind of play I'm doing. And then, when I call you in because of that, I want you to look and act the way your headshot suggests.
Then, as a final point, it's all about aesthetic and that's not something you can pre-empt. If I'm casting a romantic lead, the chances are high I'm going to cast someone who fits into my understanding of "attractive". I'm an eyes man, so my attention is going to be drawn to the eyes. Other people look at the hair, the mouth, the shape of the face. It's impossible to predict. It's something interesting and eye-catching. If I knew what that thing was though, then I'd be writing my book on How to Get Ahead in Acting!