We use improv in all kinds of fun ways. Sometimes it's to invent or discover new things, sometimes it's to weird out the other actors, and sometimes it's to create a sense of fun, to find new things inside the scripted lines.
It's funny, because '1600 Penn' was the first time I really started to read the reviews, because I am an executive producer and I wanted to see what people were enjoying and not enjoying as a means to an end, right?