TrevorMark saidMy advice to you is...
You are already generally in the requirements to be a model... but you'll have to express different things facially. It's become a "who can portray this look the best" industry rather than just pretty people, especially if they plan on doing creative headshots.
This, totally, especially on your first photoshoot (or "test" photoshoot).
Basically, the most "alive" and communicating thing on a model in a picture is the eyes, and more generally, the rest of the face. They wanna know if your eyes can look alive in a picture, or not. If they can communicate through paper and ink, you know?
There's three ways to communicate with the eyes : think about something/someone you love, think of something that makes you happy (even or something), or think about sex.
Britney Spears is the best example of "dead eyes" syndrome I could find.
as opposed to
(don't pay attention to the smile, that's kindof a bias. Just look the eyes, and you'll notice a difference)
Practice different position and learn about the multiple angles on your body, you will need to WORK THEM ALL. That is neck (side to side, down and up), hips, shoulders, elbows, ankles, feet, knees, etc. After a 3 hours shoot, you really feel like there's no position you haven't done yet, and still, the photograph is asking for more.
Mouth is another thing : you can be smiling, or mouth completely close (not really recommended), or slighlty opened (but not smiling).
That's what came to my mind for now
Good luck !