The suicide of Robin Williams has prompted a lot of discussion. This is one of the best pieces on the topic I've seen. The point is, I think, true for performers of all sorts (see this reflection on the death of Marilyn Monroe) but perhaps the contrast is most glaring for comics.
And if this talk has you thinking of offing yourself -- don't. Hey, stealing material from another comic is no way to behave, right? You can get help and get through this. Contact the National Suicide Prevention Lifeline.
Because Cracked is driven by an army of aspiring comedy writer freelancers, the message boards are full of a certain personality type. And while I don't know what percentage of funny people suffer from depression, from a rough survey of the ones I know and work with, I'd say it's approximately "all of them." So when I hear some naive soul say, "Wow, how could a wacky guy like [insert famous dead comedian here] just [insert method of early self-destruction here]? He was always joking around and having a great time!" my only response is a blank stare.
...The medium has nothing to do with it -- comedy, of any sort, is usually a byproduct of a tumor that grows on the human soul. If you know a really funny person who isn't tortured and broken inside, I'd say either A) they've just successfully hidden it from you, B) their fucked-uppedness is buried so deep down that even they're in denial about it, or C) they're just some kind of a mystical creature I can't begin to understand. I'm not saying anything science doesn't already know, by the way. Find a comedian, and you'll usually find somebody who had a shitty childhood.
...But I guess my larger point is that if you know somebody who might be at risk but you've been denying it because they're always smiling and joking around, for the love of God, wake the fuck up.