I tend to make my players stay very concrete and observational. Absolutely no causal explanations for the apocalypse allowed, gently and politely denied.

-What do you see when you look to the horizon at sunset?
-What smells waft to you on the wind?
-What wakes you from your daily slumber?
-What's the best meal you've had this month?
-You've just caught yourself staring off when your eyes suddenly focus...on what?
-A chill went up your spine, you know a threat is just outside of your senses...what is it?

and so on.

The players responses stay more or less independent at first, but quickly start to intertwine, begging for explanation.
The inputs are like points of information, with each player developing hypotheses to thread the points together in explanation, but I won't let them go there.
Playing into the confirmation bias, players will then add more content consistent with their own hypothesis and the group starts a slow build towards something for us all to discover together.
The momentum is established once the players start stepping on one another for air time. I go Socratic, picking at details to help me with threats and such.
Sometimes, I need a clock to cut off the rolling content flow that occurs. Maybe an hour or so.
Then it's time to discuss how the group fits into this world they've created.

Fun thing is...I never explain what or why the apocalypse is. It just is and no one knows.

No problem at all. Hope it helps.

One note worth offering for anyone else reading along, the game page on Roll20 begins almost completely blank for the first session. Yes, formatted to receive input from the players as we play, but blank of any fictional content. To better illustrate, I've copied to the game linked in my earlier post both the before and after (first session) game pages.

The difference between the before and after game pages is fictional content. As the players and I play the game during the first session, I take notes on what they discuss and ask questions if there are any pauses in the conversation. It's very Socratic. Post first session, I start adding information to flesh out threats, wonder about relationships within the content the players offered (triangles are my favorite), and jot down honest questions I'd like to see answered through play (aka: stakes).

To your point before, the game will remain up for a while this time. The game page therein illustrates how I use the 1st edition First Session sheet.

Sorry to have missed you. I was playing with my kids. You have GM permissions and can see all my usual prep in pink. Happy to talk you through.

An example or two for you, just in case I'm not in Roll20.

1. Player says his character has an ex-wife named Linda. So I place Linda on the first session sheet. I wonder where to slap the name "Linda" and settle on despair and envy as a reason she'd be his x. She's a bit out of the picture for now, so I slide her name away from the center.

2. Then another player says the group has to hunt for food. Furthermore, its a way to be cool in the group (the mighty hunter and all that). So I put "hunting" down near hunger and ambition, but close to the PCs, since they have to take part.

The idea is to honor the input of every player by their inputs being placed on the circle, and corrupt each by association with the apocalyptic themes.

Reading through the 2nd edition and prepping for play, most of the changes are spot on and very helpful.

One change, however, is harshing my mojo: the lack of apocalyptic themes on the first session worksheet. While having spatial relation in the threat boxes is cool, the lack of themes to which I bend the players inputs during the first session is not as good.

FYI: when I MC, my first session sheet notes are open to my players. I type their inputs for all to see and associate the input with the themes on the 1st ed first session worksheet. The effect achieved is the players twist their inputs (or at least approve of their modification) so as to be corrupted by the apocalypse. It's a subtle and powerful effect for the opening sessions of the games I MC.

Question: what am I missing in this shift from themes to spatial relationships? Usually Vincent is two or more moves ahead of me, so I'm concerned I'm missing an important concept.

My intent, baring helpful advise from you all, is to continue to use the 1st ed first session "circle" whilst everything else will be 2nd ed.

I have been getting some great feedback and little interest in this homebrew over at the Hero System forum. It surprises me how many simulation-oriented players enjoy a little narrative in their gameplay. :)