This may come across as a bit crass but guys, please take away a lesson from this death, okay?
If your friend comes to you and says he's wrestling with depression, don't go "but you're always laughing and smiling!" Just... believe the guy, and be supportive. Often it takes so, so very much to just do that step of reaching out for help and admitting you need it, and it's devastating to have it brushed away so casually.
A lot of times depression and comedy go hand-in-hand. When you're depressed, a lot of things about life are thrown into a harsh light. All those little things that don't make sense but we go along with anyway become super obvious, because to a depressed mind, that often comes across as all the things making it so the world is stacked against you. Those same things you notice are also things that make damn good targets for satire.
Even moreso, depressed people get into the habit of playing a role. Of doing things on rote. If your friend is always joking around, he's probably going to continue joking around when he's depressed... but he'll be doing it to put on a show for those around him, not actually because he thinks his jokes are funny.
Depression is a hell of a thing, guys. When a friend reaches out for help, just help them. You don't know until you ask just how much of their laughter is a disguise they're wearing for your benefit. Of course, you are fully within your rights to draw some boundaries and make it clear your friendship isn't as in-depth as a trained counselor etc etc., but that's a whole 'nother post, really.
This is tragic and sad and bullshit and hurts like a punch to the gut, to be sure.
But maybe we can remember to be on the lookout to keep somebody from ending up in such a desperate and awful place as Robin Williams found himself in. Maybe we can just look for ways to take better care of each other.
Sappy shit, but there you go.