- the conversation is unscripted
- they get roughly 3.5 hours of footage for each episode
- it takes roughly two weeks to edit them down to the 8 to 20 minutes that air
- Seinfeld himself is in the editing suite the whole time
- Each episode costs c. $100K
- The guests are paid
- it started as an "experiment" – Seinfeld had no idea whether or not it would work
At one point, near the end I believe, Letterman remarks that there are guys in comedy who are much funnier in informal private conversation than they are in their act. This leads Seinfeld to observe that there's a world of difference between being funny and a comedy act. An act is a well-tuned machine and "being funny" is the fuel it runs on – Seinfeld's analogy.
There's also some discussion of the difference between this format and the traditional talk show, that is live and before an audience. Seinfeld argues that "get" things out of a guest that wouldn't happen in a regular talk show. In a regular talk show, with the audience, the comedian would also go for a laugh and that limits what they say.
I could go on.