Everyone seems to have an opinion about this as well. I gravitate towards horizontal shots for a couple of reasons: First, the way casting directors see headshots these days is mostly as hundreds (or thousands) of thumbnails on a breakdown site such as Actors Access. You will have to crop in your shots to fit these thumbnails, and in my opinion, the horizontal crop allows for a better close crop to your face in these circumstances. Secondly, I like the way a full size horizontal shot shows up on computer screens and fits actors websites. But again--opinions! There is not hard and fast rule, and we'll shoot some of both.