Some days you can feel completely fine, have amazing connections with people, love your work, be a completely present and loving parent and spouse. And on really bad days, it feels like the sky has completely collapsed on you and that the only logical thing to do is make the world a better place by leaving it. It may sound silly if you don't suffer, especially when you recognize much of it is brain chemistry, but it is very real to the person experiencing it.
If you know someone who suffers from depression and has communicated to you that they have suicidal thoughts from time-to-time, I strongly recommend that you let them know that you are there for them at any moment when things feel really dark. They can call, text you, get you on the computer, whatever. Things can go downhill extremely rapidly, so if you ever get a 9-1-1 from a sufferer, react immediately. Get to them in person if it's possible or get someone who lives close to get to them, don't stop communicating with them until you do.
And don't try to fix the problem, telling them it's all in their mind. Again, it's all very real. Let them talk, just allow them to express themselves in whatever way it comes from them, it might be sadness, anger, rage, pain. Let them know they are safe to communicate anything with you, even the dark thoughts. I hope none of you have to go through this, but if you do, you might be saving the life of a friend, spouse, child, parent, aunt, uncle, someone many people love dearly.