Flag private discussions as such by narrating how they are kept private

My group runs a similar setup to this where we generally assume that the actions and discussions of the characters mirrors those of the party in some way. If we're making jokes around the table, our characters are making jokes while traveling or pulled up at an inn, etc.

When it comes to discussing tactics, it's the same thing: Player A can tell Player B "Use fire on the troll to stop it regenerating!" which then is mirrored by the character calling out this info across the battlefield.

Where it can get the party into trouble is exactly what you're describing: an NPC "overhears" the party discussing how they want to proceed in a given situation.

The way we generally handle it is the party tells the DM "My character pulls the Paladin aside (or out of sight, or whatever) and quietly says I think we should pickpocket the guard to get the keys"

The point is that the party is flagging to the DM that what they are saying is meant to not be heard by anyone but those characters/players involved. It also allows for a more narrative description of events. Everyone can imagine the action of the party members stepping around the corner and huddling up.

Meanwhile, those characters not actively in the huddle can keep watch for eavesdroppers by rolling checks. This allows players not actively involved in the discussion to still participate.

Once the game is on, you are the character

In my group, we also have a concern with immersive gameplay. So, when the game starts, we become the characters. So, is reasonable that everything we say is what the character is speaking. To each other, we talk always like the character. Everything we need to discuss the system or rules, we talk to DM only. Using your case as an example, we would discuss the plan to the interrogation during the travel or request to discuss this before we arrived at the NPC's door. Any question or comment regarding rules was directed to the GM, and then we interact with each other with the information in hand. The problem seems that your GM is not even allowing a moment to interact with each other in-game. Without this, you will not be able to formulate strategies and will perform only improvised actions.

O que fazer?

The only solution to me is to discuss it. You GM needs to give you the proper time to plan actions, at least in-game. If the group cannot agree on how to play, you should consider changing the campaign. It's reasonable that you all have chance to organize between one scene and another.

Some things for the GM to consider

• A scene to plan the next step is a good chance to roleplay. We have a lot of fun making the characters discuss strategies, which can even lead to heated arguments, confrontations and drama. All those things create memorable moments for us to remember.
• To be able to plan beforehand is to act like a smart and prepared person, which is good for heroic characters. For the players, there is a feeling of being smarter than their opponents.
• You cannot stay exactly 100% of the time of the game session immerse. Immersion does not means "never get out of the character" but rather "stay in character while it makes sense".
• GM should remember that the main goal of an RPG campaign is fun and joy. People play together to create incredible stories. It should be good for everyone.

Prefácio

Okay you're going to run up against my ideas on how to run a game. Many people don't believe in OOC discussion, at all, period. They consider that meta-gaming. And while certain forms of meta-gaming is bad, I don't believe it is necessarily so.

I, on the other hand, love OOC discussion. I won't say other ways of running the game is wrong. But I certainly don't like them.

I think TTRPG is like a writing a book in real time. And no good book skips the editing process. Without OOC discussion (among other tools) you don't get this opportunity to edit your game to make it better. The example you use is a really good. A GM, no matter how good, can make mistake and forget to do this and that. Actually they definitely WILL make mistakes. An OOC discussion here not only makes sense narratively (as a group on travelling together, you ought to be discussing strategy) but also meta-narratively (the game is smoother and more fun because of it)

So here's my suggestion

Have an honest out of game discussion

If your group all believe in flexibility and editing games on the fly, tell your GM that. Be sure to respect his opinion too and try to find a compromise. Let him know it's ok to make mistakes and discover the boundaries of OOC and IC discussion as you go. No one can get it perfectly without testing it out in game. You are playing the game together and figuring things out together. It's not a job it's an adventure.

Provided that he's willing to listen, ask him "Why was he so against the idea of OOC discussion in the first place? Try to understand his motives. If he wants you to roleplay more, then instead of an OOC discussion, ask him if you guys can do a Flashback in-character instead. If he's worried about too many recons, ask him to give out limited tokens to add narrative details per session, etc.

There is a solution to every problem, provided the group is willing to find it.

But if he's too set in his way, it's usually a bad flag. Either accept that you'll play by his rules all the time or just withdraw from the game. My own personal experience is that not playing is better than playing at a bad table