I also think that Harvey Korman looked like he was doing community service when he was on The Muppet Show, I guess not everyone can be ultra-enthusiastic like the aforementioned Hamill, Denver and so on. John Cleese really looked like he hated doing the show, so I was very surprised to hear that he actually co-wrote the episode that way.
I always thought that Marisa Berensen looked like she was having a blast, especially when singing "Our House", and Leo Sayer looked like he was enjoying himself too. Lou Rawls on the other hand looked rather uncomfortable at times, almost as if he was thinking, "Okay, what am I doing here?". Maybe it's just me.
Anyway, I really enjoyed reading everyone's posts.