If we look at experienced actors, we'll see how one of the secrets of camera acting is to low-key everything. If you don't, you'll most probably overact. Mind, physically low-keyed does not mean emotionally low-keyed.
Acting in a number of shorts movies, I often came to realize during filmed rehearsals how I often tended to give a single emotion (a moment of surprise, fear, or happiness) its own body or face movement. Result: I was overacting all of the time! When I say overacting, I mean either doing something unnecessary, or being "too much". So I learned to reduce, reduce, reduce, get rid of what is superfluous (sure, I decide what's necessary once I have more or less completed the work on my character), cut and make it lean, rather than fat and bombastic.
Overacting on long shots may still be acceptable, as it might still pass unnoticed. Close-ups, on the other hand, won't be that pitiful. In a close-up, the faintest movement will appear wider than it actually is; the slightest rocking of your head, which will not be seen in a long shot, will get your face partly out of frame for a fraction of a second in a close-up. Here is where I always try to clean myself of anything that might create disturbance. It happened at times I was into a scene with lines and one take was filmed in a long shot. Right after, for the following take, the setting, lighting and everything was changed to make it a close-up of me saying my lines. That's where I stop doing what I was doing in the long shot, if I realize that what I was doing might be disturbing here, or calling the attention off what I consider important in that close-up. Close-up also means: be mindful of your voice, volume and all. I learned to work in close co-operation with the sound guy (the one working with the boom).
Michael Caine has been a major source of understanding and inspiration in my gradual grasping the mechanisms of camera acting.