Voadam
Legend
In 5e the DM can just determine an NPCs reaction or call for a roll, it is a DM call.By RAW perhaps, but this is an area where I disagree with both the words and intent of the RAW. The DM should have the same degree of control over NPCs as the players do over their PCs and be able to determine their reactions in exactly the same way.
That said, if a DM or a player can't decide how a character they control will react then they can always randomize it with a self-roll if desired. But that's entirely at the whim of the "recipient", not at the whim of the person attempting to persuade or influence; and need not be binding if a better reaction presents itself.
What this means is that if your PC is trying to talk your way past my NPC gate guard the mechanics (or in this case, lack thereof) should work exactly the same as they would if my NPC gate guard was trying to talk its way past your PC.
5e PH Page 186
"RESULTS OF ROLE PLAYING
The DM uses your character's actions and attitudes to determine how an NPC reacts.
***
ABILITY CHECKS
In addition to roleplaying, ability checks are key in determining the outcome of an interaction. Your roleplaying efforts can alter an NPC's attitude, but there might still be an element of chance in the situation. For example, your DM can call for a Charisma check at any point during an interaction if he or she wants the dice to play a role in determining an NPC's reactions. Other checks might be appropriate in certain situations, at your DM's discretion."
5e gives a DM discretion on whether to call for an ability check or not.
The specific wording of Charisma ability checks suggests they work on any subject and do not distinguish between PCs and NPCs. So PCs using them against PCs or NPCs using them against PCs is not prohibited by the wording, but they do only have enumerated examples of the typical PC targeting NPC use.
Page 179:
"Persuasion. When you attempt to influence someone or a group of people with tact, social graces, or good nature, the DM might ask you to make a Charisma (Persuasion) check. Typically, you use persuasion when acting in good faith, to foster friendships, make cordial requests, or exhibit proper etiquette. Examples of persuading others include convincing a chamberlain to let your party see the king, negotiating peace between warring tribes, or inspiring a crowd of townsfolk."
There is discretion and ambiguity here so a DM can justifiably make a ruling either way, they can justifiably treat PCs and NPCs the same or differently under these rules sections of the PH.
A DM would be justified under these rules in never calling for an ability check, or in calling for one whenever anybody tries to persuade another. There is discretion and justification to treat PCs the same as NPCs for Charisma ability checks or to treat them differently.