There are a lot of models and scenes that I've watched and this is as you say "I totally buy that in a scene" when they appear to be caught up in the heat of the moment!
That is also where I tend to see scenes as" a porn shoot" when the action lacks any real emotional reaction from a model.
I was a Big Fan of
Tyler Evans I would go back and watch his scenes in a batch. There were scenes in which you could tell that from his body language and facial expressions, he was either not into his scene partner or into the activity itself. He was just doing his job and what needed to be done in the scene. But, there are several scenes in which he's fully engaged, his look, his breathing, his body language all indicated that he was either feeling good doing what he was doing or.....maybe a little turned on by the scene partner!
This is only one model and example but, this is the kind of : organic, spontaneous, real, snippet that I see when I watch scenes. It's not something I expect or get in every scene but, it's the kinds of things that allow me to buy into a scene the most. This isn't something (in my opinion) that can be directed into a scene despite what people may suggest. And both the achieves as well as many current scenes, are full of these moments....if you allow yourself, you'll find them