How to handle a PC wanting to interrupt a spell cast in a non-combat situation?

Question

A warlock and a monk are talking to a NPC. Suddenly, the warlock gets angry and fires an Eldritch Blast at the NPC. The monk recognizes the situation and tries to prevent the warlock from casting.

What can I have them do to resolve their conflicting actions?

(When I say that the monk recognized the situation, I mean just that. The monk and warlock have fought alongside each other for some time, and well, warlock wasn't attacking in a stealthy manner—he yelled "You maggot!", extended his hand forward, and so on—basically what the monk had seen countless times before.)

Eldritch Blast calls for a spell attack roll, for the sake of discussion we can assume it would hit. The monk is probably using his reflexes (so, Dex) to prevent the spell from hitting the victim. We can assume that the victim doesn't move at all.

Idea 1: This just became combat, so rewind and roll initiative. Whatever the outcome, the warlock and the monk have already declared their actions. Depending on who acts first either the warlock fries the NPC, or the monk stops him.

I have a problem with this - the warlock is using his spellcasting abilities, and he could have Dex of 3 and still be a formidable and very quick caster. Using initiative (dex based) is completely nonsensical.

Idea 2: The warlock's spellcasting ability (Cha) vs. Monks Dex. Makes some sense - he is casting, the other one is reacting with his body.

Do the rules specify what to do in this situation? If not, what do you think is reasonable?

Answer 1

Dexterity includes reflexes.

Despite the usual definition of "dextrous" - in Dungeons and Dragons it is expanded to measure a character's reflex speed as well. See this quote from the PHB:

Dexterity measures agility, reflexes, and balance.

So your interpretation of the the warlock - "even with a 3 DEX he could still react quickly" - is not how the rules are set up. A low DEX, by definition, implies a slow-reacting character.

Out of your choices, I would go with the first - as it is more suited to how the rules would interpret this situation. However, you are the DM, and you could house-rule this situation however you wanted since it's player-vs-player combat more than anything.

The Case of the Monk's Rationale

Other posters have mentioned the requisite "how does the Monk know the Warlock is going to attack?" In many situations they are correct, only the most paranoid layman would attack someone just for "wiggling their fingers" at them. However, if our Monk is either knowledgeable in the subject (Arcana) or is especially adept at reading others' intentions (Insight, maybe Perception), then he would get a chance to recognize our Warlock's hostile intentions. One more possibility is that these two gentlemen are longstanding party members. One could attempt to argue that this Monk, regardless of his knowledge in Arcana, has been fighting with this Warlock for long enough that he knows generally what an attack -in this example, a very common one at that- looks like coming from the Warlock and would thus have valid reasoning for anticipating the attack. These arguments would fall on the ears of the DM, and he would weigh them accordingly.

How would I handle the situation?

There is definitely precedent here for this just being a skill check between the two players, instead of full-on combat. Maybe DEX vs DEX?

Alternatively, depending on your players and where you, as the DM, want the story to go you could simply say "The Monk slaps you in the face as you try to cast, annoying you and breaking your concentration." If the NPC is foolish, he may not even realize what has happened. Nothing has changed, no lasting damage has been done, but this possibly-overly-brash action has been halted and given everyone involved a "second chance" at the scenario. Note: this is entirely a DM-fiat/ignore-the-RAW approach. It depends on how strict your group is with the rules.

Answer 2

When the warlock "began" casting the game moved from social interaction to combat - there are well defined rules for this (p.189 PHB):

1. Determine Surprise From your description, neither the warlock nor the monk are surprised; the NPC may or may not be at your discretion - it rather depends on how the conversation led up to the combat.
2. Establish Positions Everyone is within 5 feet of each other (unless it was a shouted conversation)
3. Roll Initiative With 3 creatures there are 6 possible sequences of initiative - I'm not going to enumerate them here.
4. Take Turns See below
5. Begin the next round Enough said

There are very limited ways for the Monk to stop the Warlock from casting the spell and all of them depend on the Monk beating the Warlock's initiative, they are:

1. Asking him not to - this requires the Warlock to agree to the request
2. Giving the Warlock a condition that prevents him from taking an action, these are:
   1. Incapacitated
   2. Conditions that include Incapacitated (Paralysed, Petrified and Unconscious)
3. Preventing him from using Verbal (e.g. silence spell) or Somatic (e.g. see 2 above) components. There are no rules for covering his mouth or grabbing both his hands but if the Monk chose one or both of these I would make a ruling on how it would possibly work
4. Killing him (possibly an over the top reaction)

In addition, the monk could move between the Warlock and the NPC, providing half-cover. Of course, there is nothing stopping the Warlock moving before casting to get a clear line of sight again.

The important point is: in 5e, there is no way to stop a person from completing a 1 action or 1 bonus action casting time spell once they have started it. Longer casting time spells require concentration which can be broken but Eldritch Blast is a 1 action spell - if the Warlock uses his action to cast it then it is cast!

Some parenthetical notes:

1. The NPC may beat the Warlock on the initiative. If they are surprised then the can take no actions but they will no longer be surprised when it is the Warlock's turn. If they are surprised then they may (would) do something aggressive to the Warlock before he casts his spell.
2. This:

   When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn’t incapacitated.

Hostile has no clear definition but the NPC is hostile even if surprised, the Monk may also be hostile.

3. I am a firm believer in human rights; the most important being the right to take the consequences of your actions. If you announce at my table that you are attacking an NPC with deadly force, then I will ask you once "Are you sure?", if you say yes then that is what you do and the consequences that flow from that are yours and yours alone.

Answer 3

Magic in D&D is not of the silent, mental kind.

To cast a spell, you will need gestures, words and material components. Not all of those for every spell, but at least one of those.

Eldritch blast uses (V)erbal and (S)omatic components, which means words and gestures. As soon as something requires gestures, for example pointing at the target of your eldritch blast, dex based initiative makes sense.

Answer 4

In 5e, there are explicitly almost no ways of interrupting a spell being cast -- even if you're an enemy in melee combat with them. And it's that much harder if you're in a conversation not expecting them to cast.

There's a feat, mage slayer, which lets you reaction-attack attack someone when they cast a spell within reach of you, like an attack of opportunity, as was the case in previous editions. But even if it does damage it doesn't necessarily fizzle the spell, only if the spell requires concentration and they fail the concentration check. That suggests that people without the feat should not be able to do that.

Both for balance and flavour reasons, it's probably sensible to use that as a default for 5e. So by RAW, I'd say, "no, it's too late to physically stop them".

Initiative doesn't help, since even if you retroactively assume combat turns, the warlock presumably acts without warning. After all, the same would apply to a physical attack, there's no way to stop someone except stopping them first.

However, I'd maybe allow the monk to shout "no" and try to grab the warlock's sleeve, and up to the warlock if it was in time.

If this is going to happen a lot, I'd maybe allow a skill check (perception? dex?) to sense a spell about to be cast. And maybe even a stiff skill check or attack roll to duplicate the Mageslayer feat. As long as it's an option in a niche situation where it would be unrealistic to have no way to prevent a spell, and doesn't affect normal combat balance where spells are a scarce resource but wizards are limited by HP not concentration checks.