John and Merlin both showed up saying, "We wanna kill the fuck out of something." I obliged with a swarm of giant rats, dredged up from the subterranean sludge by the thrashing tentacles of the ziggurat monster. They knew they were getting close to the monster, and this was the final room to overcome.
Hard hint, easy hint
The room included two tablets with Orc runes containing instructions for feeding the monster. "Feed it the dead or the healthy; if it swallows someone while they die, that will put it to sleep." This is the same info I earlier tried to impart in less clear fashion to Merlin through the vial of memories.
This is definitely a pattern for me when I have a solution in mind and the players haven't yet come up with another one to supplant it:
First, give really obscure data that only reveals the solution with some deduction and insight. Never enough to prove the right approach, but enough that, if the players do put it together, it's coherent enough to be remembered and pursued. When the players latch onto these, they feel smart and clever. It's my ideal, but it would cease to be as fun if it happened every time.
Second, back up the first data with something similar but more direct. Remind the players of what they might have deduced earlier. Take a detail that was uncertain and make it definite. If they figure it out at this point, they usually still feel pretty clever, and a little more confident that they're correct. (Although that varies among players! John's more likely to think he's got it right away, for sure; Dan's more likely to say "close enough to pursue"; and Merlin's more likely to remain cautious and demand more certainty.)
Third, hand them the answer, but make their characters earn that discovery with guts, sweat and blood. That's what the rats were for. Interestingly, the guys decided, "Orc writing! Must be bad news!" and smashed the tablets to bits. Fortunately, since their characters couldn't read Orcish anyway, I had another delivery method in mind for the info (shrieking processed by Dan's "protection from sonic attacks" ward), and I just ruled that this would still work with the tablets smashed.
When is it cool to dissolve our fingers with acid?
There was an odd exchange that I managed poorly but it worked out fine because of shared expectations.
Early in the game, Dan was extremely cautious while crawling through a tunnel toward a foul-smelling vent. He pushed a torch along in front of him, always careful not to get too close to the vent. This was fun for me, as I'd created this vent as filled with an acid mist that would dissolve anything. I got to describe a little torch sputtering and hissing, and coming back a little weird-looking.
Later, after killing the rats and approaching a doorway clearly connected to the same smelly vent, Dan just said, "I tie a rope to my torch and lower the torch down the vent." My brain went, "The acid mist melts you! The final door to the great beast is guarded, foolish mortal!" I needed to say something quickly, but I didn't want to do that classic GM obnoxious move of assuming a player character is stupid. SO I imagined him approaching it slowly and being able to react quickly when the acid mist hit the first part of his body. Envision the torch-holding posture, I said, "There's a sizzle and you lose feeling in your thumb and index finger."
He quickly pulled back, and asked me to describe what it looked like. I narrated that his gauntlet had crumpled in a bit, and the rope and torch were fine. It was basically an "oh shit" moment on my part. I'd intended for the mist to disintegrate anything that touched it. But now, having already melted off Dan's fingers, I wasn't sure whether it was plausible to say, "The torch melted too, but you didn't have time to react." I couldn't tell whether the torch would have melted before his fingers, or what kind of warning it might have given him. So I changed the effect to dissolving living tissue; and then I decided that his armor responded to the melting hand by crumpling in, which probably doesn't make sense when you think about it.
Wrangling the fiction to remove a character's digits? Nightmare, right? Nope. Dan's response was, "Fuck! Well, fair enough, I'd stopped being cautious." We both took his change in approach from tunnel to doorway the same way.
I don't really have a conclusion about how to reconcile careful arbitration of actions that might or might not maim characters with the needs of dramatic narration. But I can say that a good understanding about who's responsible for establishing expectations for character behavior gives you a lot more leeway.
What have we been maneuvering around all this time?
We played through the characters descending a staircase into the tablet room, and the room filling with rats, and the characters moving toward the stairway so they could retreat up it. Only at this point did anyone discuss whether the stairs were surrounded by walls or not, which was vital to how defensible they'd be (rats climbing up the sides is bad news!). Dan had assumed the stairs were walled while John had assumed they weren't.
I hadn't thought about it, but the plan to bottleneck the rats on the stairs sounded good to me, so I said they were walled. This is not at all how I'd instruct a new GM to run Delve; winging stuff this way can violate the players' trust in the GM's impartiality. But once everyone in the group does trust each other to value fictional consistency and plausibility over personal agendas, then winging it is much safer.
That said, the ideal solution remains for the players to ask for more info when entering a new space, or for anyone at the table to anticipate a relevant factor and ask about it before the action's fully underway.
Formalize environmental questions?
I've written down, "I survey the area," "I look for _," and, "I try to judge _," as reminders for players to seek info about their characters' surroundings. I've never tried to enforce use of these specific phrases, though, responding equally well (as GM) to questions about "what's there?" or "are there rocks?" etc.
After playing Puppetland and Kagematsu at Camp Nerdly, and getting great atmospheric mileage out of very strict narration constraints, now I'm re-thinking this. Mandatory use of these key phrases could better force players to view the fiction through their characters' eyes. From that perspective, I find that color and details become more interesting, and forgetting to ask what kind of staircase you're on is rare.
Table of fallback options
I wrote up a list of common things the characters do, and jotted down constructive options for each. That way, if the players got stumped on, say, making a plan, they could look at the Making a Plan options and say, "Oh, hey, Pick a Top Priority might be a good call here!"
No one looked at the sheet all session. But this was a high-action session. A session with some non-urgent moments and decisions to be made might be a better test.