Check out the two videos below - the first one was an interview of Emma Watson in 2007 when she was promoting Order of the Phoenix. The second one was her recent interview for The Half-Blood Prince. Notice how Letterman asked the same questions like a senile old man.
I've never found Letterman to be a good talkshow host but this discovery has lowered him in my eyes - it's one thing to be a bad interviewer but to be lazy is probably one of the most damning sins in the field of journalism and the like.