As many of us know, working with models can be a real challenge when it comes to the visual result of your concept. It's both a challenge for the photographer and the model and the key to the best result is communication between the two parties. Be friends with your models, know them, be relaxed and play.
That being said, after searching for imagery of people involved in different activities, I noticed there are two major types of model expression: models looking at the camera while involved in an activity, and models that interact naturally with the object of their activity. The first category may lead in numbers, but the latter has more sales. That's why I think it's very important to realize that a natural looking attitude has a better sale potential.
I will briefly emphasize on the natural appeal of a photo, that can be achieved when greater attention is paid to the context and to how the model should relate to it.

For example, mechanics at work: in the first image, the models focus on their job, interact with that specific environment and it all looks natural. In the second image they pose rather than work, not to mention that looking directly at the camera while working with the wrench can be hazardous, if not plain dangerous; the image looks a bit artificial by comparison.


Another example, young women lifting weight, exercising: The first model is clearly focused on her work, mostlikely paying attention to her trainer, while the second is rather posing and relates directly to the photographer, as well as having a pretty awkward working position.
The first shot looks natural, giving the impression that the photographer randomly and discretely snapped a young woman who was unaware of his presence. The second image shows a direct relation between the model and the camera, with less regard to the workout itself.