iserith, an interesting thread.
I hope some questions are OK. And a comment at the end.
Thanks, and sure!
A common GMing pitfall can be setting stakes which make sense and create drama in the fiction, but are hard to follow through on if the player fails the roll. In this case, what happens if the shark drags Rosemary under? Presumably not the PC's death by drowning. Does she just lose her backpack?
I would likely say she's pulled under a certain distance, then ask what she does. The next looming threat could be that she's in danger of being pulled even deeper if she does nothing or fails at something. Or maybe more sharks are approaching.
If she lets go of her backpack (which likely contains her spellbook?), she can get away. Or maybe she tries to stab the thing with a dagger or the like (a roll) which would probably have the result of the shark letting her go. Others who haven't yet been in the spotlight might also try to intervene at this point.
What is the basis here for the GM determination that Chuck didn't tidy up after himself? It does not seem to have been part of the adjudication of the check to disarm the trap.
Part of the DM describing the environment. If I needed to justify myself, I could say that in his haste to disarm the trap and in Lack-Toes rage in destroying the trap, it was dropped, left behind, slipped out of his kit, or whatever. Maybe it's not even his upon closer inspection - it belongs to Poncho Cloak (who is found dead later). Plenty of fictional justification to be had here to set up a dramatic moment.
These sorts of cases, where taking a fairly simple step gives advantage to the check, can give rise to all the PCs doing it. Especially because, in the fiction, we can easily imagine it becoming clear to the others that Cow's walking stick or Rosemary's wet handkerchief are providing a benefit.
How do you impose some sort of cost or other rationing constraint on these sorts of actions, and their imitation by other PCs, within the D&D framework?
After the players have described what they want to do and I'm setting the stakes, they're effectively locked in - they've already taken the action. The roll follows so I can narrate the results. There is no "space" between action and result to modify their action... in most cases. In some cases, it might be reasonable.
The insight check raises the possibility of the other PCs trying too. How do you handle that? Presumably as some sort of group check, but how do you adjudicate the outcome? And handling it as a group check does seem to presuppose that, in the fiction, the PCs are debating among themselves whether or not the salamander is being truthful, which goes somewhat against your "player autonomy with respect to PC belief" directive.
In this case, it was one player describing a goal and approach to sussing out the salamander's truthfulness. I established in the stakes that upon failure, the salamander is inscrutable. That would apply to everyone, unless they can change up their approach to the problem. If someone approached the situation the same way as Sacred Cow, they'd just fail automatically, no roll.
I've never had a situation where a group Insight check came up, but I think that it's certainly possible. In this case, if the group check succeeds, I will state the truth without reference to anyone's beliefs as I did above: NPC is telling the truth or NPC is lying. A player who botched his or her check might then decide to believe the opposite even though he or she goes along with the group's decision (which might be worth Inspiration). If the check fails, then either the NPC is inscrutable or perhaps, if progress combined with a setback is in order, they determine the NPC's truthfulness, but the NPC suspects that they know and takes appropriate action.
I'm not sure about the "too many ability checks" dimension to this, but I completely agree about the "failure to engage the other players (via their PCs)". If you put the pressure on the other players to have their PCs say something (eg an NPC asks them a direct question, or the social situation is heading in a direction that they don't want it to) then in my experience they will declare actions for their PCs even if CHA is not the PC's best stat, just as the player of a wizard will declare actions for his/her PC if you tell him/her that an Indiana Jones-style boulder has just started rolling down the corridor, even though STR and DEX probably aren't that PC's best stats.
Agreed. With regard to "too many ability checks," I've seen a lot of players expect that any dialogue they offer is going to result in some kind of roll and since they're playing with the fighter with the dumped Cha and no social skills, they clam up. I try to create an expectation that it's entirely possible that you can engage in social interaction and never have to make a roll. Or that if you want to act in support of the lead character in the scene, you can potentially give that character advantage when he or she does have to make a check.