Actors are taught when the other actor in a scene is speaking, they should give their focus to the other actor and let the other actor have her or his moment. In editing, we can cut to the actor listening if we want but why would we? On camera, someone just listening isn’t interesting. A character struggling with her or his own emotional experience while another character is speaking is what creates cinematic power on camera.
What the camera loves more than anything is a character caught up in his or her own emotional experience. This doesn’t mean actors shouldn’t be generous to their fellow actors. It means when the creative work of you and your fellow actors is done and the camera starts to roll, the actor in front of the camera has an artistic responsibility to make every moment, every frame all about and only about what her or his character is thinking and feeling. In other words, the film camera wants you to think like a movie star. That way when the director is editing the film, the director can cut to your character at any point during a scene and your character will be carrying the emotional story and commanding the camera’s and the audience’s attention.
In life and on set, you can be the most wonderful, generous, genuine artist in the world but when you walk the dozen or so steps to your mark in front of the camera, the camera wants you to become a movie star. After we call cut, you can go back to being the wonderful, generous, genuine artist with whom we love working but while you are in front of it, the camera wants you to think like a star.