To be successful and look professional in front of the camera, you must be able to hit your mark every time. Your mark is the place where you are supposed to stand in order to be in the right position in the frame. Sometimes, this will be marked on the ground with a piece of tape and other times the director will simply tell you where to stand. Practice until you land on a predetermined point smoothly and without looking down.
The eyes are very important in film acting, especially during a close up. Be aware of what your eyes are doing and control extraneous movements, such as excessive blinking or eyebrow wiggling. Film legend Michael Caine has a great tip about the eyes. He suggests that during your close up in a dialogue scene, you should always look into your scene partner's eye that is closest to the camera. Focus on that eye, rather than moving between the two.
Theater acting requires a fair amount of bombast and over the top energy in order to come across as truthful. This is due to the size of the room and the distance between the actor and the audience. In film, the audience is much closer. To hit truth in a film performance, keep every aspect of your performance within the realms of reality. Avoid "acting" a certain way, and instead focus on your objectives and play the scene truthfully.
In a theater, you always know where the eye watching you is, because there are a hundred of them right off the front of the stage. In film, the eye is the camera, and you need to know where it is at all times. This is especially important in multiple camera TV shows. You need to be aware of the angles during each take. Learn to be aware of the camera and to know when it is your turn to take the focus and when you need to stay out of your partner's way.