Can a character provide help with a Persuasion check

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In page 175 of the player's handbook under section Working Together it reads:

Sometimes two or more characters team up to attempt a task. The character who's leading the effort–or the one with the highest ability modifier–can make an ability check with advantage, reflecting the help provided by the other characters. In combat, this requires the Help action.

A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves' tools, so a character who lacks that proficiency can't help another character in that task.
Moreover, a character can help only when two or more individuals working together would actually be productive. Some tasks, such as threading a needle, are no easier with help.

Anyone can attempt to persuade a character alone so the first requirement is fulfilled, but would multiple individuals working together to persuade someone be productive?

Best Answer

Yes they can

Anyone can attempt to persuade somebody alone, even if they are not proficient in Persuasion, so anyone can help with the check.

There are also ways one can think of how a second participant can make the persuasion more productive. For example, they can give subtle signs to the speaker if they are going in the wrong direction (the classical "kick under the table"), they can bring up secondary supporting arguments, they can confirm that what the speaker says makes sense etc.

Social Proof is one of six well-established¹ methods for persuasion: when more other people seem to believe something, it is harder to not accept it.

You as the DM might ask the player how his character is helping, what they are saying to improve the attempt, to allow them to help. For social checks there is always some tension when you ask for "playing it out", because that is not required for example for physical checks like Athletics, and so demanding it shouldn't disadvantage players that picked social skill proficiencies vs players who picked others. However, when you allow anyone to try this, there is no such specific investment of character development resources needed, and asking for some creative input should be fine.

¹ Robert Cialdini, Influence: The Psychology of Persuasion

The Short Answer (TL;DR)

You need Thieves' Tools to pick a lock but you don't need to be proficient with them.

The Long(er) Answer

You (usually) need Thieves' Tools to pick a lock or disarm a trap BUT you (probably) don't need to be proficient with them (maybe).

Exactly how you are meant to use Thieves' Tools isn't 100% explicitly and unequivocally covered by RAW (~~yet~~ still) so this is my interpretation of those rules based on the rules I've found.

Part 1: Do you need Thieves' Tools to pick a lock?

This part of the question is easy: YES

Thieves' Tools are (surprisingly) a tool:

Tools

A tool helps you do something you couldn't otherwise do, such as craft or repair an item, forge a document, or pick a lock.

-PHB p.154/PBR p.50

This is pretty explicit and there aren't any contradictions to this general rule elsewhere. One potential problem is how to deal with improvised Thieves' Tools. The DMG has given us some good basic rules to deal with improvised Thieves' Tools under the Sample Traps section:

Collapsing Roof

This trap uses a trip wire... A successful DC 15 Dexterity check using thieves' tools disables the trip wire harmlessly. A character without thieves' tools can attempt this check with disadvantage using any edged weapon or edged tool. On a failed check, the trap triggers.

-DMG p.122

It's probably allowable to use improvised Thieves' Tools (in most situations) providing you have something sensible to make them from (at your DM's discretion). In addition, any lock picking or trap disarming check made with improvised Thieves' Tools should be made with disadvantage (if it's allowed at all).

Part 2: Do you need to be proficient with Thieves' Tools to use them?

This is the trickier part of the question as there are a few contradictions and unclear rules for this in the PHB/DMG. An important thing to note though, is that there isn't a section in the PHB/DMG that handles lock picking or trap disarming specifically. This means that a lot of it comes down to you and your DM's interpretation of any references to using Thieves' Tools.

There are a good number of references to Thieves' Tools in the PHB and DMG so let's look at the relevant ones and see what they say:

Equipment

The item descriptions for the Lock and Manacles both reference Thieves' Tools (though interestingly the Hunting Trap doesn't).

Lock. [...] A key is provided with the lock. Without the key, a creature proficient with thieves’ tools can pick this lock with a successful DC 15 Dexterity check.

-PHB p.152/PBR p.49

and

Manacles. [...] Each set of manacles comes with one key. Without the key, a creature proficient with thieves’ tools can pick the manacles’ lock with a successful DC 15 Dexterity check.

-PHB p.152/PBR p.50

Both of these listings could be interpreted to say you have to have proficiency to be able to pick the lock. However, as Cthos pointed out, a (fairly strict) reading doesn't actually exclude anyone who isn't proficient from making the check.

This could mean that a creature without proficiency in Thieves' Tools faces a DC 15 Dex check, a DC 15 Dex check with disadvantage, or even just a higher DC Dex check instead.

Tools

The general Tool rules and the Thieves' Tools description both point towards proficiency being non-mandatory:

Tools

[...] Proficiency with a tool allows you to add your proficiency bonus to any ability check you make using that tool.

-PHB p.154/PBR p.50/DMG p.239

Note: The DMG wording is infinitesimally different (and essentially identical) to the rules in the PHB/PBR

Since the phrasing of this is non-exclusive, like the equipment listings, it implies that you can take a non-proficient check with any tool.

Effectively, adding a proficiency bonus to a check (or not) is meant to simulate the "real world" impact of being proficient (or not) with whatever is involved in the ability check you're making.

Thieves' Tools. [...] Proficiency with these tools lets you add your proficiency bonus to any ability checks you make to disarm traps or open locks.

-PHB p.154/PBR v2 p.51

This description gives absolutely no indication that you need to be proficient to use Thieves' Tools, which I think is one of the strongest indicators that proficiency isn't required.

This is (quite surprisingly) the only main point of reference across the rule books. In addition, it also indirectly implies you can make a non-proficient check to disarm traps or open locks without getting your proficiency bonus and without disadvantage.

Ability Checks

Working Together Sometimes two or more characters team up to attempt a task. [...] A character can only provide help if the task is one that he or she could attempt alone. For example, trying to open a lock requires proficiency with thieves’ tools, so a character who lacks that proficiency can’t help another character in that task.

-PHB p.175/PBR v2 p.59

This is the second strongest case for proficient use only - after all, it is explicitly stated that you need proficiency to open a lock. This is the only reference to outright state this though and it does contradict the implications of all previous rules.

It could be that in this case "specific beats general" and Thieves' Tools specifically need proficiency whereas other tools generally don't. However, this would make them the only tools to need proficiency.

I also think this could very well be errata, missed in the final edits, based purely on where it is (i.e. this isn't in the description for Thieves' Tools).

Other Dexterity Checks. The DM might call for a Dexterity check when you try to accomplish tasks like the following:

Control a heavily laden cart on a steep descent

Steer a chariot around a tight turn

Pick a lock

Disarm a trap

etc...

-PHB p.177/PBR p.60 (v3)

Again, this is a reference to taking a Dex check with no other qualifiers requiring proficiency (this was pointed out by Jason K in the question)

Mapping a Dungeon

Locked Doors

Characters who don't have the key for a locked door can pick the lock with a successful Dexterity check (doing so requires thieves' tools and proficiency in their use).

-DMG p.103

Ok, this provides the strongest argument for needing proficiency to use Thieves' Tools (and it's a strong argument one at that). This quote is pretty unflinching in terms of RAW and could be a deal breaker. However, as Jason K has pointed out, it is in a section that isn't about tools so could potentially be errata (and it isn't backed up by any other statements in the DMG.

Traps

If the adventurers detect a trap before triggering it, they might be able to disarm it, either permanently or long enough to move past it. You might call for an Intelligence (Investigation) check for a character to deduce what needs to be done, followed by a Dexterity check using thieves' tools to perform the necessary sabotage.

-DMG p.121

Here, again, is another reference to making a Dexterity check without explicitly requiring proficiency. This could have just been left out if it's already assumed you do require proficiency but that's not really stated as a normal assumption.

Twitter

Finally, for completeness, here's the tweet and reply between WaxEagle and Mike Mearls (co-lead designer of D&D 5e).

Important: I have stolen this directly from WaxEagles's answer purely for completeness. The effort on WaxEagle's part should be acknowledged here and I take no credit for this quote.

Conversation

WaxEagle: Could I get some clarity on thieves tools and lockpicking. Do you need the tools? Do you need proficiency?

Mike Mearls: anyone can use tools, prof bonus adds if proficient. No tools, DM can say no check or disadvantage.

Again, this is a pretty major point for allowing non-proficient lockpicking (or trap disarming) but I believe Mike has previously stated that Jeremy Crawford is the "rules guy" so it's not absolute (unfortunately).

Conclusion

After reading into the PHB and DMG, finding these rulings and taking them all together, my final assessment is still the same: I'd suggest that we can (and should) make the assumption that characters can attempt to use any tool they want.

Ultimately there is nothing physically preventing any character from picking up any tool, weapon, musical instrument, or set of armour they aren't proficient with and then trying to use it.

That said, a non-proficient character doesn't get their proficiency bonus and may also take an additional penalty, or have disadvantage (see non-proficiency with armour), so they're definitely less likely to achieve the outcome they want if they do try something unfamiliar.

If you're going for a "Gritty Realism" theme (DMG p. 267) to your campaign, then yeah, you could (house)rule that you need to be proficient and no amount of Dexterity is going to help without some kind of training or practice.

And that does kind of hold up in real life - ever tried to pick a lock before? However, I think a lot of people would agree that these guys, your characters, are heroes! I'd prefer to give them the benefit of the doubt, at least where RAW is concerned.

It would be great to see a definitive answer from the development team of 5e (Jeremy Crawford, Mike Mearls, & co.) but I'm not sure how likely that is. We shall see...

To restate my conclusion:

You do need Thieves' Tools to pick a lock but you don't need to be proficient with them.

Epilogue: How this works in practice

If you're still here and agree with my assessment, all this effectively means there are really only four scenarios you would come across when picking a lock:

You have Thieves' Tools and are proficient with them. You can pick the lock and get to add your proficiency bonus to the (Dex) check.
You have Thieves' Tools but you aren't proficient with them. You can still pick the lock but you don't get to add your proficiency bonus (since it's a bonus you only get when you are proficient with something).
You don't have any Thieves' Tools but you improvise some (with your DM's approval). You can still pick the lock but it's probably with disadvantage. If you have proficiency with the tools, add your bonus. If not, good luck with your roll!
No Thieves' Tools and no improvised tools. Take a strength check to throw the closest party member through the door or crowbar the lock. Basically, look for another way to get past it because you can't pick it.

[RPG] It seems like every skill check should always be made with advantage due to the ‘Working Together’ rules. Is this accurate

This is how skills are supposed to work!

If you are in a situation where there is only one person doing something, and they are rolling a single skill check, then yes, this is how it's supposed to work. Giving help is a natural thing and should be used in situations like this. There is no reason to prevent it unless the task is clearly something that's not going to benefit from someone else giving you assistance. There are some things you can do to limit it.

It's also worth noting that helping can often save you some table time. As a AceCalhoun points out in the comments, in many cases what happens if you don't help is that everyone in the party tries their hand at the task. This behaves very much like advantage, but with a slightly lower overall modifier (because most likely you'll have one character who is good at a task and the rest that are lower). So Working together only raises the change of success slightly and consumes less table time in these cases.

Be a bit more stringent about what you allow for assistance. Is coaching stealth really all that helpful? do you really want someone looking/talking over your shoulder while you're picking that lock? Evaluate situations where characters attempt to aid more carefully.
Have more than one thing going on at once. If all the characters need to be stealthy, they can't be helping each other. And if you need two arcane characters working on the sigils on opposite sides of the room, maybe they have to choose which one gets help from the third (or don't have anyone to help at all).
Make things take multiple rolls and limit helping on all of them. Maybe the first roll the wizard can be helped, but after that he's on the other side of the trap, or arm deep in the sigil or something to where additional assistance isn't going to help him.
Figure out how to inflict disadvantage for the task. Maybe there are mitigating circumstances.
Create a distraction. A rogue can't help the wizard if he's busy fighting baddies. Make some skill checks happen in an occupied room. Make completing the skill checks the win condition rather than defeating the enemies.

The basic crux of all of this is that helping is supposed to a mundane task that provides advantage. Yes, that's a huge deal, but it also doesn't stack with other things that give you advantage and it can be easily cancelled by disadvantage.

So get creative! Build some situations into your adventures that prevent your heroes from helping each other (or make the opportunity cost higher). But don't do it all the time, that might get tiresome. Adventurers like to help each other out, let them, but don't make it easy all the time.