I got to know many stage actors, who interpret correctly, with a good diction and stage presence but when they are placed in front of the camera it seems that they are just starting in the world of interpretation. They overreact, position their voice and exaggerate their facial expressions.
A talented actor will be able both for stage as for screen, the most important thing is to understand the cinematic technique.
I have prepared the summary of the most relevant aspects that should be taken into account:
1. Start having a very clear idea that for screen you have to act in a totally contrary manner tan for stage. Instead of expressing you have to internalise, instead of exaggerate you have to simplify and instead of remember the next sentence, you have to forget it.
2. Spectator on screen always views the whole body of the actor, but on screen there are a lot of close-ups. For that reason on a large screen, your image frequently is bigger than your natural size, and the minimum nuance of each of your gestures is magnified.
3. The voice in the theatre has to reach the last row (for example 30 metres). In the film, the spectator is where the camera is (for example in 1 metre distance). For that reason the volume of our voice has to be the same as we would talk to the person who is at 1 metre distance from us.
4. In the theatre the actor has the liberty of movement, and in spite of having references or marks, if he doesn´t fit to them, it is not very important. On screen the actor has to control his movements all the time, if the actor is only some centimetres out if the established mark, it may appear not perfectly in the camera.
5. The gestures in the theatre tend to be more exaggerated, on screen any grandiloquent gesture looks ugly and even in bad taste. The camera doesn’t permit misleading, an excesses results redundant.
6. In the theatre the actor interacts with other characters, receives their stimulation and energy. On screen the actor normally play alone, at least other character appears in the camera. For that reason the actor has to imagine all that stimulation.
7. When the actor plays for the stage, he starts and ends, lives all the process of the character. In the film, the actor starts shooting the end of the film, later shoots the beginning and at last the middle of the film (the film is shouted in fragmented way because of the needs of production), for that reason the actor doesn´t experience that process and has to place his emotions depending on the scene.
8. For the screen is important to know, pronounce and interpret well the text, for the film is more important the simplicity, you should simplify in order to be closer to reality.
9. On stage you can offer an interpretation, but in front of the camera is better to have an experience.
In conclusion, either on stage or on screen, the same ingredients are needed, but in different proportions, and you as an actor, should feel the same, the only difference is the manner that you express it.