The Letterman interview will probably be the same thing as Leno. What else that's interesting could they talk about that wasn't mentioned in Leno?
Well, it depends on the person conducting the interview (it can make a world of difference, in my opinion). People like David Letterman are somehow able to evoke certain things out of celebrities when they're on his show (probably because he's a comedic genius). It's always entertaining to watch how he interacts with guests, ESPECIALLY beautiful women. If anyone has seen even one of Natalie Portman's many (seven and counting, I think) interviews done on Letterman, they would know what I'm talking about. He's a great interviewer (which is why I am so looking forward to seeing her on HIS show).
And, yes, they do sometimes ask the same or similar questions, but that's not always the case. Dave has a knack for getting those "funny stories" out of his guests. Conan too.
So, you see, like I said before, it does make a difference. A WORLD of difference.
(P.S.) Can you tell I hate Leno?