What Expression Should You Have in a Headshot?
The answer is: It Depends.
For business headshots, expressions showing you smiling or a confident look are always good. Having too serious or intimidating a look may be off-putting to some people. And stay away from smirking. It tends to convey cockiness.
For actors, however, you might need several headshots with different expressions to show your acting range to casting directors. If you're going for a comedic role you're going to want a headshot that connects for that role. Same thing if you're trying to land a role in a drama. That comedic, smiling headshot is not going to fit the role.
So think about what you want to convey in your headshot expression before stepping in front of the camera. Discuss this beforehand with your headshot photographer in a pre-shoot call. That way you'll both be on the same page for the expressions you want out of your headshot session.