Stephen Sondheim appeared twice as a guest on The Colbert Report and I find those chats fascinating. I actually found all the interviews Mr. Colbert did on that show fascinating because they were done with almost no prep or pre-interviews, and Colbert not only had to improvise his end of the conversation, he had to do so in character. A lot of folks who are considered expert at improvisational comedy marveled at his ability to do this.
He would tell the guest before the show something like, "I'm playing an idiot. Don't be afraid to push back and treat me like one." This worked surprisingly well when he had on a guest with many areas of disagreement…and it was probably even harder for him to play that moron when the guest was someone he clearly admired, such as Sondheim.
Sondheim's first time on the show was 12/14/10 to plug the book Finishing the Hat and you can view that interview here. Notice how much these guys liked each other and how Sondheim really seems to be enjoying himself.
He came back on 11/30/11 to promote the follow-up book and again, you can see how well these two men got along…