I'm more or less in the middle of a crisis atm, so I'm probably a good person to ask.
But I wanted to start by addressing what to if you see someone you don't know, with an obvious serious mental illness, who appears to be in crisis (e.g. hallucinations, delusions, suicidal ideation etc). Please call your local emergency hotline and ask for a welfare check. They won't get in trouble and you won't get in trouble, but your could save someone's life's.
As for me, most recently I've been diagnosed with a bunch of different stuff, some vague some less vague, specifically: Autism (or Aspergers or whatever you want to call it), Bipolar II with Obsessive Compulsive tendencies (I forget the exact phrasing), more or less a type seasonal depression (I get fucked up in winter), insomnia and chronic nightmares which isn't an actual diagnosis but Parasomnia covers a bunch of different things.
As for how/why I'm in crisis, I would prefer not to go into a bunch of details there but I more or less tried to kill myself, ended up in a horribly over medicated mess where I could barely remember anything or make sense of anything but I was still deeply deeply broken (sad isn't the right word). For me when I get to the point that I want to end it, it feels like I've spent my whole life holding my breath and I just want to be able to breathe again and ending it is the only way I can do that.
Anyway after all the rambling, to answer your actual question, for me personally it's mostly just to talk. Most of the time I just like to get everything that is in my head out, but in an actual conversation not necessarily just a rant. And it will be a long convoluted conversation that will seem to be going nowhere because the first part is building up the confidence to say 'Hey I want to top myself, got any thoughts on that?'.
It takes a little while to establish not just that you are a safe person to have that conversation with, but that you are in the right mood and that it's the right time and the you're actually going to listen.
The people who listened to me over the last week, and acted on their gut instincts probably saved my life. So yeah, conversation is key.