CGI is done after the film is done. It's through the computer. Most of the film is not computer-generated special effects. Most of it is that creature that is in the room with you.
“If you are a novelist of a certain type of temperament, then what you really want to do is re-invent the world. God wasn't too bad a novelist, except he was a Realist.”
“Fear not, we are of the nature of the lion, and cannot descend to the destruction of mice and such small beasts.”