Green screen (sometimes blue screen or yellow screen) acting is something actors of the distant past would have excelled at, as it demands of the performer the same kind of disciplined imagination that turned the stage of a Greek amphitheatre into a heavenly kingdom, or the "wooden O" of the old Globe into the battlefield of Agincourt. The bare stage and the green screen studio share the wonderful, exhilarating vastness of creative possibility.
One is presented with a blank canvas on which to paint. The background will be determined by the script, and designed or generated on a computer. It is the actor's task to create a believable performance in the imagined setting. The possibilities are endless, determined by the requirements of the text of course, but with very few limitations of any kind. One can be inside or outside, in a huge castle or a tiny room, or swim in space or fly through the sea and not get wet. Usually it will just be you, your co-performers and the screen –so you have to create the dimensions, the feeling and the atmosphere of your surroundings. Some actors dislike this, preferring the firm realism of objects and surroundings that are actually there. I myself find the challenge of green screen acting to be hugely liberating. Your imagination is fired up and sharpened; you go back to using the inner eye of a child. You are told the vague limits of the space –there is a door here, in the background there will be a window etc– but you yourself have to create the sense of being in that space. You may have to react to something that isn't there –that will be generated on a computer afterwards, so you need to be disciplined and focused, and see the height, colour, texture of everything around you that isn't there. Your creation of the imaginary world around you is what will (hopefully) make the setting believable. Though technical wizards can do wonders with the backgrounds, your performance has to be utterly believable for the thing to work –and the hardest thing is: you just don't know if it's working. You just have to trust your instincts and your imagination. And THAT is really what makes acting exciting.
