Extreme Characters is a concept the book introduces. Mike correctly states that it will not be useful for most project but it is worth knowing about. I agree. But it got me thinking. Wouldn't Extreme Characters fit into Soap Opera Testing perfectly.
Cohn sites Djajdiningrat, J.P., W.W. Gaver and J.W. Fres, "Interaction relabelling and Extreme Characters: Methods for exploring aesthetic interactions." Symposium on Designing Interactive Systems 2000, 2000:66-71. See poster for the article.
A 5 minute google didn't turn up much of anything detailed on Extreme Characters other than what Mr. Cohn wrote/teaches. Food for thought and grounds for further research.
(Cool. I bought the Kindle ebook and read in on my laptop at work or on the Kindle at home. It is also sitting on my iPhone should I want to make a quick reference during a stand up.)