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.
“Be not entangled in this world of days and nights; Thou hast another time and space as well.”
“Life is hard. Then you die. Then they throw dirt in your face. Then the worms eat you. Be grateful it happens in that order.”