Not to knock Mamet (I also have a quote from that memo on my FB page, cause he's awesome) -- but I don't wholly agree with the 2 people talking about a third being totally useless.

It often works well for a character introduction. Have two people talking about a character the audience hasn't met yet to setup an expectation. This can create tension when we finally meet the character. It can also create comedy if you go against type.

Any other time, I agree though. It comes off soap opera-y usually.

James, you are right. In theory. In practice, you are correct. However it's important not to miss the big picture per Mamet's memo and that is this: For Hollywood writers, exposition is always a pain in the ass, especially so on the TV side of things. How to handle it effectively. How to determine how much is too much / how much is too little. How to convey it in fresh, different ways. And the thing is, no writer really likes exposition, it's something to be gotten through in order to get to the fun character and action stuff.

And yet exposition is absolutely necessary. Hence the continual conundrum: How to handle exposition in the best, most efficient and entertaining way as possible.