Usually it would be aperture priority (f/2.8) in a situation where I know what the light is so I can set the ISO and leave it there. I develop with DxO so I am not worried about noise or the shutter speed too long but I do worry about hitting 1/8000 sec -- so usually it would be a situation where the lighting is predictable.
The autofocus can be set in a mode where it will reliably lock on the subject's eye. I would demo how you have to be a certain distance to get a headshot, since it is a prime lens, if they are too far away I don't worry too much because modern cameras have a lot of pixels.