I think the goal is to make a well written scene seem like it's improvised and/or to come up with things that you find in the room that you couldn't have known until you get into the real situation, just try to improve things as you go along.
Robert Downey, Jr.I saw School of Rock, and I was like, why haven't I worked with Richard Linklater already? Then by the time I got him I was like, I'm really pissed off I feel like you owe me some retroactive swag. He gave me the 10-year anniversary "Dazed and Confused" T-shirt, which I still wear with relish.
Robert Downey, Jr.