In the year and a half I was on SNL, I never saw anybody ad lib anything. For a very good reason - the director cut according to the script. So, if you ad libbed, you'd be off mike and off camera.
Harry ShearerA movie script more than anything else is a plan of action for the crew. Everybody in the crew looks at the script to see what they're going to do. It has to contain where you are, and how many people are there, and what they do, and what time of day it is, and what time of year it is.
Harry Shearer