Can you use your action to interrupt another player's action from earlier in the same round?

Question

I play in a game when everyone plays the way they want on their own turn : saying to another player what he should do is forbidden. Your turn = your action.

I think this is pretty fun and it adds a lot of stress in difficult fights since we can't coordinate ourselves in real time, but sometimes someone makes a mistake that could be avoided in a roleplay way.

Is it possible to use our own action to interrupt another player's action that was played earlier in the same round?

Answer 1

In between your own turns, you can only interfer with the ongoing combat via a reaction.

The section The Order of Combat of the PHB outlines the fundamentally turn-based nature of combat in DnD. As is explained afterwards under Your Turn (PHB, page 189), you usually act only on your turn, unless you act as a reaction:

A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's.

So, unless triggered by circumstances defined by your own class features, feats, spells, etc., you cannot act outside of your turn. There is a way to define such a trigger yourself, though.

If it is your turn and you expect an ally to do something on their next turn that you disagree with, you could set it as the trigger for a readied action to interrupt it.

If you expect your ally to do something specific, you may decide to wait for it and react to it when it happens. The Ready action (PHB, page 193) enables you to do this:

Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.

First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it. Examples include "If the cultist steps on the trapdoor, I'll pull the lever that opens it," and "If the goblin steps next to me, I move away."

When the trigger occurs, you can either take your reaction right after the trigger finishes or ignore the trigger. Remember that you can take only one reaction per round.

For this to work, you'd have to anticipate and describe with sufficient precision an action an ally would take. A trigger as vague as "one of my allies does something I don't agree with" will probably not be allowed by most DMs. If your DM accepts your trigger, you can then describe what you will do if the trigger actually occurs. But keep in mind that

• a readied action replaces the action you would take on your own turn (you can still take a bonus action on your turn),
• it is limited to either movement or an action (not both and no bonus action),
• if the trigger occurs and you use your readied action, you can no longer use your reaction for anything else
• if the trigger does not occur and no other circumstances let you use your reaction, you have essentially wasted your complete contribution to this round (unless you have multiple actions due to Action Surge (PHB, page 72) or Haste (PHB, page 250) or anything similar).

Taking into account your playing style and your efforts to avoid meta-gaming, you would of course have to declare the trigger and the readied action to your DM in secret, so that the other players do not know your intentions.

If an ally has already taken the action to which you object, only powerful magic can make it undone.

Under certain circumstances, you could use the Wish spell (PHB, page 288) to try to undo what an ally has done on a preceding turn. However, it depends on the DM to which extend this is actually possible. Anyway, this is hardly an everyday solution.

Answer 2

Once your turn is over, you can no longer act unless you have a specific ability, typically one that involves you using your reaction, that allows you to act outside your turn.

Every creature can do this using the Ready action, and this might be exactly what you're looking for.

From the SRD:

Ready: Sometimes you want to get the jump on a friend foe or wait for a particular circumstance before you act. To do so, you can take the Ready action on your turn, which lets you act using your reaction before the start of your next turn.

So, to summarize you could very well interrupt an ally's action by using the Ready action as your action, declaring that you'll do something at the same time or in response to one of their actions.