Background Information:

System is CoC, the game is a multi year long campaign, set in the 1920s. The Players character has a chemistry and firework background. The character has no strong weapon/combat skills (except for brawl) so the Player likes to get creative to be able to help in combat.

One Player in our group often has elaborate and almost crazy ideas to achieve their goal. For example they would ask if they could buy prunes and aloe vera at a local shop. The GM asked them to roll "luck", it was a "extreme success" so they were able to buy both, and a bottle of berry licqor. The player put all together with some things they had in their bag, and gifting it to a Barkeeper (framed as an apology) who punched them in the face earlier.

After this took place the GM asked the player what they were trying to accomplish. The player responded that is was a strong laxative, and they should hurry out of town before they have to live with the "shitty consequences".

They also made makeshift explosives, that look like nailpolish, or cigar tins.

The GM is aware that they are doing something, with the things at their disposal, but they are usually not aware of what their plan is, until they use their items.


Is it fair of a player to do something like this, without the GM knowing where it is going?

How can a fair damage roll be determined, if the GM allows the usage of those items?

  • 1
    \$\begingroup\$ For CoC, is DM the correct term for the Game Master? While I like what your player(s) are leaning into, I am not CoC-savvy enough to write an answer. We do have some folks who can, so I am looking forward to their responses. Welcome to RPGSE! 😊 \$\endgroup\$ Commented May 26, 2023 at 10:52
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    \$\begingroup\$ Dungeon master is a trademark term owned by Wizards of the Coast, so other RPGs avoid using it and use some variant of game master instead. Some do use a different term, including picking different words for the GM acronym. I'm going to revise this question here to use GM since that's clearer when discussing Call of Cthulhu than calling the role by a different game's term. \$\endgroup\$ Commented May 26, 2023 at 11:06
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    \$\begingroup\$ I believe CoC uses the term "Keeper". \$\endgroup\$
    – From
    Commented May 26, 2023 at 11:16
  • 4
    \$\begingroup\$ Hello and welcome! Is there any particular reason why the roll has to happen before the DM asks the purpose? Is it to ensure some kind of impartiality in setting the difficulty of the check? \$\endgroup\$
    – biziclop
    Commented May 26, 2023 at 12:02
  • 2
    \$\begingroup\$ @From correct, more specifically it's "Keeper of Arcane Lore" \$\endgroup\$
    – AnnaAG
    Commented May 26, 2023 at 14:07

6 Answers 6


There seem to be two things going on at the same time, one good and one not so good.

The player is trying to be inventive with their abilities.

This is great, and it should absolutely be encouraged. Any changes or discussions should focus on making sure that they don't just slap down the player for being creative.

The player is not involving the GM in their plans.

This is not in itself a problem but it could lead to problems. Essentially it puts the GM on the spot to react to an unexpected situation in real time, and the outcome may or may not be desirable.

If the GM doesn't mind being forced to improvise at speed and the players are generally happy with how the GM handles these snap decisions, this is fair.

However if it often leads to party discontent or too much pressure on the GM, with them struggling to come up with a roll for how much damage such a contraption would make, discussing a possible change is a good idea.

What can be done about it?

This sounds kind of obvious but the GM should simply ask "By the way, what do you need these items for?" or "Can you outline to me what your plan is?"

If the player doesn't want to share their plans with the other players (so as not to spoil the surprise for them), this can be done in private, by passing notes for example.

It should be emphasised that this is absolutely in the player's interest, as a better prepared GM can warn them of potential obvious mistakes in their plan (mistakes their character would not make), or just make the outcome more spectacular or narratively satisfying.

  • 1
    \$\begingroup\$ "can warn them of potential obvious mistakes in their plan". I don't play RPGs, so pardon if this is a stupid question: why in the world should the GM give the player a head's up? It's not as if IRL gods actually whisper "don't do that!" in your ear; you live with the consequences of your actions... \$\endgroup\$
    – RonJohn
    Commented May 28, 2023 at 4:55
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    \$\begingroup\$ @RonJohn it depends on the table, but I tend to think of it as giving players the information their characters would know. The GM (in many games) is the world, and the characters live in the world, so they intuitively would know the expected outcomes of many things they attempt, in a way that the actual players (who don't live in this world and also can't read the GM's mind) don't. It just helps head off disappointment down the line when the players thought the world worked one way but it doesn't. The following line in the answer clarifies this: "mistakes their character would not make". \$\endgroup\$ Commented May 28, 2023 at 8:59
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    \$\begingroup\$ @RonJohn You're absolutely right about that but this is not about mistakes in general. This is strictly about stuff your character knows but you as a player might fail to recall. If you play a chemist for example, your character will know that you can't distil ethanol to 100% purity. You as a player however may not. This is the kind of mistake your character definitely wouldn't make. \$\endgroup\$
    – biziclop
    Commented May 28, 2023 at 10:57
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    \$\begingroup\$ @RonJohn I've never played DnD or any other game with a GM, but from what I've read about it, it's not "GM vs players" - it's "GM with the players". The idea is to have a fun adventure, not a realistic adventure, and this works better when players and the GM are in sync. The GM will occasionally work out things in the player's favor, when the alternative would be a deeply dissatisfying outcome. Not always, of course. The most fun happens when the game is challenging, but not overwhelmingly so. \$\endgroup\$
    – Vilx-
    Commented May 28, 2023 at 20:25
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    \$\begingroup\$ That said, an element of surprise can also be fun for everyone. :) \$\endgroup\$
    – Vilx-
    Commented May 28, 2023 at 20:26

GMs need to know what you do!

As a player, you need to inform the GM of the purpose of the rolls you do as well as how you do whatever you want to do. Let me preface with two examples of how groups around me have handled genius ideas.

A ship in the South Atlantic, shortly before 1930, a group of explorers in Call of Cthulhu. A crewman goes mad. The doctor (me) asks the GM if they have a larger stock of Insulin on board, he'd like to administer Insulin Shock Therapy. The GM looks up, shrugs, and asks:

GM: What does it do?

Doc: By administring a heavy dose I put the patient in a coma for like... 12 hours or such so that he can sleep off his madness and doesn't disturb the others.

GM: What happens if you mess up?

Doc: If you underdose, the patient just goes numb, if you overdose they die while sleeping.

GM: Ok, Roll for it. Medicine, please.

The GM adds how hard the roll is, dice fly and... Success! The guy will be cold out from 6 PM to 6 AM. 30 RL minutes later...

GM: The maddened Crewman screams in his sleep around midnight...

Doc: I doubt. That crewman should be out like a stone, because he's in a coma... that is, unless something seriously wrong made him awake...

GM: Oh, right. I forgot you put him in coma. He's the only one not having disturbed sleep that night.

A besieged town in a low fantasy world. A well schooled mage ponders options...

Mage: I want to gather sulfur, charcoal, and saltpeter...

GM: To make Gunpowder, right? Hang on a sec. What's your alchemy skill? Also, remember that Gunpowder does not work like that in this world. The closest you get is pretty much greek fire, and the recipe for that is not just highly classified, it also includes a very specific spell. But... You can do some research rolls to get it figured out over some time.

Mage: Good to know. I think I try to figure something like that out then.

Those two examples show quite different ways to handle such a player, but they rely on two different recipes:

Get clarification first

Before you allow the player to do anything, ask for the reason and the plan. If they want to put the crewman to sleep or concoct a laxative, they have to say so first. It helps if you also ask the player to clarify what happens if they mess up with their plan.

Divert and realign

The second example was a player that was bringing in a thing that didn't work in the game world. However, the GM then (me) did realign the player's general idea of making something to defend the town into a path to get the closest similar done.

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    \$\begingroup\$ The bit about clarifying what happens if things go wrong is huge. Most ‘creative’ plans have more significant consequences if things go wrong than the plan just not working, and quite often those consequences can help the GM to both keep things balanced and maintain narrative tension. \$\endgroup\$ Commented May 27, 2023 at 20:27

Skills in Call of Cthulhu require skill rolls, and skill rolls are called by the Keeper

An RPG can be said to be a simulation of the real world - however, the arbiter of that world is neither the player nor the real world, it is the Game Master. Much as a Byakhee does not exist in the real world, yet the Keeper of a Call of Cthulhu game can use one in their game, real world chemistry does not work unless the arbiter says they do - and in the case of creating a chemical reaction, is likely to require a successful Skill Roll against the Chemistry Specialization of the Science Skill.

For the Keeper to be able to determine the difficulty of a Skill Roll, they need to be aware of its purpose - in short, without Keeper buy-in, there can be no chemical reaction caused by a player.

In general, this sounds like a Session 0 problem

The concepts you may want to cover in a Session 0 are many, but one of the most important is to ensure everyone is on the same page - do you want to play a game where the players spring surprises on each other and/or the GM? Do you want to play a game where player knowledge and skills can be used where character knowledge and skills are lacking? Communication is the key, and your player currently isn't doing their fair share of that.

  • \$\begingroup\$ Interesting points! One question (I believe the answer is: it depends on the GM but I would like to hear your opinion): A character which has a in depth backround in something (lets say chemistry in this case) and also a high skill point (lets say 90). Would that character need to roll a chemistry skill for the easiest chemistry things? Would that not be in the way of actually role playing your character? Role playing over rolling in a sense? \$\endgroup\$
    – Pudora
    Commented May 26, 2023 at 20:48
  • \$\begingroup\$ @Pudora You guessed it; It would be up to the GM when a roll is required, and for the GM to be able to make that determination, the GM needs to know what the roll is supposed to accomplish. As in your example, "create a strong laxative". \$\endgroup\$
    – From
    Commented May 26, 2023 at 21:25
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    \$\begingroup\$ @Pudora The rule of thumb is that as a GM you only ask for a roll if the consequences are meaningful. So if for example your character is performing a simple task in a well-stocked lab under no time pressure, they could try until they succeed anyway. It makes no difference to the story how many attempts they need to get there. However the same task in the field with your last batch of possibly not so pure starting materials would absolutely warrant a roll. \$\endgroup\$
    – biziclop
    Commented May 27, 2023 at 9:48
  • \$\begingroup\$ @biziclop In the lab example, the chance that an accident occurs before the proper mixture is obtained can (and IMO should) always be present. 😉 \$\endgroup\$ Commented May 27, 2023 at 10:44
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    \$\begingroup\$ @KorvinStarmast Fair point but I was talking about the example offered above, i.e. "the easiest chemistry things". Stuff like recrystallising and purifying aspirin for example. The chance of an accident is always there but is so low that dice rolls can't express it and simulating it would do nothing to advance the narrative. \$\endgroup\$
    – biziclop
    Commented May 27, 2023 at 10:49

You have been overly generous with successes.

The aloe-prune liquor isn't going to be magically successful. Realistically, you would have to consume a lot of prunes and aloe to have explosive effects, otherwise you simply have slightly softer poops. A prune-aloe slurry would also be hard to mistake for whiskey. Making this work should likely take a whole series of rolls: first a relatively difficult chemistry(?) roll to make some sort of laxative with characteristics anywhere near booze. Next a social roll to convince the bartender that this mystery bottle is apology booze. Finally, you'd probably need some sort of easy medicine roll/skill level to predict the effects (likely just a slightly urgent trip to the toilet).

Depending which version of Call of Cthulhu you are using, you should likely be applying penalty dice to some of those rolls.

With explosives, you should consider the difficulty and effects. Making an explosive that looks like nail polish is probably just a regular success. Making a stable explosive that looks like nail polish is probably a hard task. Having made it, you probably need to actually do something with it, which may be its own task. Depending on what time, materials, and workspace you have to work with, you should probably look at hard or extreme difficulties (and penalty dice if applicable).

Explosives also aren't magic.

As Trish points out in comments, default Cthulhu is in the '20s or '30s, and explosive technology is still immature (and dangerous).

Dynamite is absurdly powerful in CoC. it does 4d10 damage within three yards (dice halved every additional three yards). That will simply kill most entities. Unfortunately, it will simply kill most characters even more easily. Failing an attempt to throw dynamite or to rig it gets very dangerous for the user and anyone around him. Estimating quickly, a cigar box is usually about 6x6x1.5 inches inside, which is inconvenient for full sticks of dynamite, but it's reasonable to handwave that. Say you can stuff it with about 8 sticks worth of dynamite. Fusing it is going to be the exciting part: if you mess that up, you and anyone within about 60 feet are likely just gone. If you are throwing the box like a satchel charge, you will be dead (can't throw that far). If you are setting it as a booby trap, you likely need a combined explosives/mechanical repair roll, and it might reasonably need to be a hard roll (don't fumble!).

On the other end of the spectrum, the nail polish bottle is a lot less dangerous. It's also a lot less dangerous: that's about a half of a blasting cap's worth of explosive, so it's about 1d10 of damage within 1 yard (assuming it's some sort of non-electric cap, and that the character can chemistry up a useful fuse for it). That's a much more reasonable level of risk, and he is likely to even survive a failed role. The entities he wants to use it on are also likely to survive.

One final note: this is an example of a question where the system makes a significant difference. Call of Cthulhu is a grinding, punishing system, and it is simply difficult to make zany plans work. If you were playing e.g. Trail of Cthulhu, this sort of play experience would be much more normal and expected (see: What is this story I recall about an extremely long ever-changing character backstory?)

  • \$\begingroup\$ This is really insightful! Thank you for adding the bits about the damage. \$\endgroup\$
    – Pudora
    Commented May 27, 2023 at 22:46
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    \$\begingroup\$ Remember that CoC is often in the 20s or 30s - the chemistry of explosives is much less developed, especially stable explosives. \$\endgroup\$
    – Trish
    Commented May 28, 2023 at 1:22
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    \$\begingroup\$ I like this one: the answer shows knowledge of CoC and exactly the kind of arguments I’d use as Keeper to reign this in, and why if would be much smarter by the player to check before trying any of this. +1 \$\endgroup\$ Commented May 28, 2023 at 5:03

This PC is making life harder on themselves

It falls to the Keeper to decide how things that are not explicitly covered by the rules work out in the heat of play, or to decide on how hard it is to use skills like chemistry to create effective poisons or explosives.

For the record, I’ve witnessed groups taking out a whole cult in an underground lair by piping propane gas, which is heavier than air in through a window, and throwing a burning torch down there to blow them all up. This was great fun for all involved. But it’s just as possible that such tactics, if they work too well, get boring or unbalancing pretty fast, and that the Keeper for that reason rules it does not work, or is super hard, or is detected early, or backfires.

Especially if they do not have a lot of time to assess the long term consequences, some Keepers will tend to err on the side of caution in such ad-hoc adjudication, which is entirely their right.

So, more than a question of fairness, it seems to me it is a question of self-interest for this player to check with the Keeper in advance and out of game how well something could work, or how hard a check it will require. This, like the game rules, gives them a better yardstick to decide if it is worth trying — and maybe, a bit more time to explore and negotiate it with the Keeper, too.

If they spring it upon the Keeper in-game, they have to live with whatever decision the Keeper takes then an there.


From my experience (both BEING this type of player and DMing them), here's how I see it:

Practically speaking, unless your player is an actual mechanical engineer, almost all "creative" solutions fall apart if you pry into them hard enough: if such well-founded "exploits" were possible using hard science then there would be mainstream analogues in real life, in which case the player is not actually being all that creative and likely just googled a method. I remember being asked about the details of a paraglider made from the local birch fronds and devolving into science-speak almost immediately. If you want to spend the time, you can probably dissect a solution a player gives if you think it's completely ridiculous.

Ultimately though, the goal of a DM isn't to make the campaign realistic, it's to make it fun. I think a creative solution should be judged by a combination of how much effort the creator put into devising it and how much fun its application will be for the players and DM. This litmus test is easiest when a player presents a homemade pipe bomb or a method to hotwire a vehicle: they didn't put in the mental labor to devise such a solution, the application is straightforward and offers a power-up rather than a plan, and so they shouldn't reap proportionally unbalanced benefits. On the other hand, I've had players test the different densities of inert gases they had gathered from an alchemist NPC and use them to displace the poisonous gas filling a room instead of going through the standard process of finding the right lever or killing the right enemy; if it's well thought-out, relies on simple principles rather than elaborate movie-science, and doesn't completely nullify the difficulty of their pursuit, then go for it. You can even add in extra obstacles that complicate the deployment of their solution so it's still a challenge, just not the one they were expecting.


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