Warning: Mamet's guidance is written ALL IN CAPITAL LETTERS, so it's like he's screaming at you, which is a bit hard to take. Perhaps he just wanted to be DRAMATIC. In any case, I advise copying the text into a document and changing it to lower case. Your sanity will thank you.
If you're not in the mood for a lecture and would prefer a laugh, here's author Lynn Coady's advice for writing fiction, from making sure to include something that will offend your relatives to avoiding writing about symbolic dogs -- or dogs of any kind.