1) How does a session get there?
2) What does the moment-to-moment "gameplay" look like?
It's so improvisational it's hard to explain, but:
1) This is episode 27. They've been exploring this place, the Endless Imperium, a city the Undead have built to govern themselves in a civilized, if horrifying, society. Early on they were hired by a lich to collect "things," and they've come to trust and be interested in her. Their last fetch quest netted some points they are curious about - embedding a phylactery in a living creature instead of an object, to start. They go to her with questions. And I think this is where it's so interactive - I riff on their CURIOSITY. I don't lore dump on them - they can play this as superficially as they want it, but they just keep digging deeper.
1a) One of the characters is a chronomancer, and one is an echo knight, so we've played around a LOT with what happens when you interfere with a moment in time (chronal shift/personal echoes). Are those broken branches of the timeline? Moments that didn't happen? Moments that COULD have happened? And I've kind of had in my back pocket: I think this is not the chronomancer's first time in the Imperium. I think maybe her FUTURE self has already been here.
1b) They have another contact who is a wildly powerful chronomancer. Every time they go to his studio, they see potential versions of themselves in mirrors. So we've got a running theme of how there is more than one path anyone can take at any time. So I weave him into both the player's story, and the lich's story, because they love these NPCs and they're important to them.
1c) The conversation could've been ten minutes - "hey here's your thing, what's our next thing?" But I just invited them to ask questions and they couldn't stop pulling on the thread. They wanted to know more about WHY they were sent out to get these things, what that meant, where they stood in the politics of the Imperium, and because of their rolls, they kept successfully getting more information, which lead to a deeply philosophical conversation. And they were playing off each other - one question would inspire another would inspire a conspiracy theory (WHICH WAS TRUE) and so on.
2a) A bit of light roleplay to get there - a weird incident in the street that gives another character a moment to learn about themselves. Then they arrive and the NPC answers their base questions. Gameplay is a TON of rolls based on what works in that moment - arcana checks to grasp the magic theory the NPC is spinning, history checks to recognize backstory, insight checks to see if she's being truthful or bullshitting them, persuasion checks to dive deeper into what she knows. The fighter of all people rolled two crits on a persuasion with disadvantage and that told me the NPC opened up to them - you honor the dice roll. He asked the right question in the right way to get her to want to tell them more.
2b) I let them tell me what they want to roll. What do you want to know? What skill do you use? Does anyone help you? Is there anything you do to sway the conversation (upping or lowering the DC)? I'm reacting to them, too, both with their rolls and with their responses.
2c) Twice, someone just posited a theory that was true enough I wanted it to be right, so I said: sometimes a character just intuitively knows what is true. No dice roll needed. You found the answer.
I ended with just saying "I GUESS this is D&D?" because we were as much collaborating with emotional storytelling beats as we were rolling dice. We run a LOT of combat but I'd say we have a session like this once every eight or ten sessions where it's just alchemy. It's a balance of their curiosity and me having a loose story that I can tweak and tailor on the fly based on what they want to learn. These are the sessions where the group chat goes NUTS for three hours afterward as they talk about what they learned and what to do with it.