A good headshot presents the real you - as simply and clearly as possible. Theatrical headshots let casting agents see instantly whether you are the right "type" for a role they hope to fill. These call for a serious - but not melodramatic - expression.
Commercial headshots, on the other hand, usually include your most pleasant smile, and answer the quintessential question on the casting agent's mind: "Can this person sell floor wax?"
Corporate headshots can be of either type, depending on the image you wish to project.
Portraits can be anything. And they should, too.