Here's the current question and answer...
So, I ask again: what kind of a person becomes a professional comedian? Answer: one who can't stand to do anything else; one who NEEDS that audience approval so badly that he or she is not completely a person without it; one who only comes alive--quite often--when performing.
The first thing you learn about famous comic actors, once you've spent some time around them offstage, is that they are among the most morose folks you will ever meet. There is one giant exception to this rule: Robin Williams (see my earlier question/answer in the archive area about my favorite interview). All of the others mentioned above and virtually anybody else you would care to suggest--the Saturday Night Live crowd, past and present, the In Living Color crew, the Kids in the Hall collection, and anybody who has ever gotten an HBO comedy special. They tend to be shy and quiet at best--like Jon Lovitz and Damon Wayans--or moody and defensive--like Janeane Garofalo and Bill Murray.
My greatest disappointment in this area was Gary Shandling whose HBO show is my favorite source of comedy on TV--also the most dead-on look at the world of entertainment ever shown. I interviewed him and ended up reassuring him halfway through that his answers were not really as boring and uninteresting as he feared, and that he didn't need to have snappy one-liners in response to every question or nobody would be interested in anything that he said. He became so depressed with his own "performance" that I left the room feeling like I had ruined his day by doing the interview.
If you're reading all of this and saying to yourself, "now, hold on, I saw such and such on Letterman and they seemed really upbeat, funny, and secure," then I have to burst your bubble quickly. These high profile national shows are used as performance spaces by comics, very much including the conversation with the host. You are NOT seeing anything approaching their real personality in that setting. You're watching a prepared performance.
I've found that this slightly paranoid attitude decreases each time you interview them or spend some time with them in other settings. Familiarity, in the case of comic actors, breeds security. It is the lack of that security that made them comics in the first case.