When we're working on camera in class we sometimes fall victim to a tendency that has us plowing through the scene and, in the process, editing our performance out of the story. My advice is always to take your time. Let what is going on moment to moment affect you and then allow that to be the impetus for your reaction.
Another concern is our ideas and preconceived notions of what is expected of us (usually by no-one but ourselves) and how to play that. We can become tunnel-visioned in our pursuit of an objective or getting our message across that we forget that there is a living, breathing, feeling human being in the room with us that is being affected by our behavior and that in turn is having an effect on us and, possibly, (most likely) if we are connected to one another this has the potential to open us up to other feelings about them and ourselves and where the scene wants to go.
When we are working against these two conflicts a self-preservation mechanism kicks in that wants to keep us safe and so we behave accordingly by pulling back and using the words of the script as a security measure against anything harmful that can befall us, and in life in most circumstances that may be fine, but in Film and Television we do not have that luxury. There is nothing gratuitous in the 120 minutes or less within which the lives of complex individuals are thrown together. This is true of all genre in both mediums. As closely as we identify and empathize with the characters we take on, as much as their feelings and thoughts resonate with us we have to on some level recognize that the world they inhabit is heightened in reality compared to ours because they don't have a 100 years of screen time to live out their experiences. They have 120 minutes or less. Nothing can be taken for granted and moments must be lived out fully.