I don't mean to define green screen, but really to remind myself what the technology means for writers and actors.
Take a look at this promotional video for a green screen company:
Their remarkable work reminded me that you can't trust anything you see on prime time TV. Some of the clips are from shows like Ugly Betty which is not known for being special effects dependent. But it obviously is!
What this means for writers is that nothing should restrict your imagination, especially budget. You don't really know what it will cost. If it makes story sense, write it and let the production manager make you change it.
What this means for actors is that your use of imagination becomes that much more important. All those acting exercises actually become on-set requirements since you won't necessarily have the real location to inspire you or evoke feelings for you.
When I did the movie Mission to Mars,
I was astonished at how realistic the space station set was. Every button worked and made some light go on. The galley set was stocked with real NASA astronaut food. They could have used packets of cat treats because no one would ever know--except the actors. Naturally it enhanced my feeling of being on a space station--and hopefully helped the performance.
But they probably don't need to go to the trouble of constructing something as elaborate ten years later, so the onus would now be on me to imagine all the stuff that they built when we shot that film.