A few actual, non-hyperbolic years ago, I said this:
Yes, having an odd ability score as an ability damage buffer is a type of metagaming. But it’s also a way of defining a character. As in, “Rock Hardslab is so strong! He can even take a hit from a wraith and barrel on, unharmed, where lesser action heroes might falter!” I could have a whole other post someday on how selling something the right way can be the difference between rejection and applause.
This is that post.
Selling is, I suppose, a form of reskinning. Usually reskinning takes rules for something that exists, like a wand of fireball, and using them for something that does not exist, like a rocket launcher. Selling is like reskinning but for descriptions; you still have a wand of fireball, but yours uses green fire. The rules are the same and everybody in-character knows exactly what’s going on, but you’ve described it in a more memorable way. Think of it as a sentence. Your character is a noun, their actions are a verb, and selling is an adverb. There’s a huge difference between “the bard grinned wickedly” and “the bard grinned sheepishly.”
Consider Power Attack. The description of the feat in the Pathfinder Core Rulebook is just “You can make exceptionally deadly melee attacks by sacrificing accuracy for strength.” It doesn’t say how you lose accuracy or gain strength, so that’s entirely up to the player. Most players I know assume it means they’re swinging wildly, putting as much force as possible behind every blow and hoping one connects. But mechanically, they could also be adjusting their grip so they gain more momentum behind every blow but have less control over where it lands, which changes Power Attack from the random attacks of a desperate brute to an intentional decision by a trained veteran. Or, it could be that they’re going for a decisive blow, targeting vital areas even if they’re the most protected parts of the target’s body, which means it’s the sadistic action of an assassin or a wise fighter’s attempt to end the fight quickly. All of these are, mechanically, Power Attack, and the game treats them all the same way. What matters is how each description affects the perception of the character, both in-game and out of it.
Background actions do the same thing, often in punchier ways. If we know a character is putting all her strength behind every swing, she could be doing it for any number of reasons. But if her player says she’s laughing maniacally as she does it, in a single action we get a fairly clear picture of the character in this moment. If she’s instead moving inexorably forward with steely-eyed determination, the character is completely different. Again, the numbers are exactly the same. Even the actions are the same. It’s just the presentation that’s different, but that presentation matters so much.
Where this works best is when several instances of selling all compliment each other. Take our green fireball from above. That’s cute, and it’s a fun visual. Give a sorcerer that fireball, add a green magic missile and a light spell that glows green, and now you have a theme. The players and other characters starting thinking of this sorcerer as the Emerald Mage (if you like epithets) or the Green Wizard (if you like Gauntlet). All of their battles now have a unique visual and they can build a reputation based on it. And if an enemy shows up and starting slinging green ice spells, that’s half a story arc right there. Even the 3E Player’s Handbook got in on it:
You can call your skills, feats, and class features whatever your character would call them. Lidda, the halfling rogue, talks about “footpaddin’” rather than about “moving silently”, so her player writes “Footpaddin’” down on her character sheet to stand for the Move Silently skill. Ember, the monk, calls her Move Silently skill “Rice Paper Walk.”
In rare cases, selling can provide more than flavor. When the selling begins to affect the character, for good or bad, that’s when it’s really taken hold. In my first campaign we had a character, Gary “Smasher” Stubblefield. He was a rogue/fighter who specialized in charging and grappling, which was a lot to process for somebody who was only fifth-level but we were young and we went with it. Near the end of the campaign we had this exchange:
Gary: I want to attack that guy, and I have higher ground.
DM: Okay, but you’re standing on a crate, and you use unarmed strikes. You’re going to have a hard time reaching him without going prone, unless you kick him.
Gary: Of course I kick him. I’ve been doing kicks exclusively this entirely campaign.
Gary: I said in the first session I always kick with my unarmed strikes.
DM: I don’t remember you saying that.
Gary: But I did.
Other PC: I also remember this.
DM: But you’re a grappler! You grapple with arms! You haven’t described your normal attacks as a kick once!
Gary: My charging attack is named “flying kick”!
DM: So is Liu Kang’s! He still punches!
In the end he made the attack and whiffed anyway.
While this might be a weak example, in that the character description absolved a character of an arbitrary penalty rather than giving him a proper benefit, it’s not hard to think of similar situations. A neat-freak wizard who always uses prestidigitation to clean up after himself might be harder for the enemy ranger to track. The easily-distracted bard might not be looking where she’s going, which means she doesn’t have to make a save when the party turns the corner and finds a medusa. Selling is the difference among “I walk down the corridor”, “I sneak down the corridor”, and “I crawl down the corridor, inch by inch, looking for traps.” A sufficiently clear or thorough characterization should rightly give a character some bonus on appropriate checks (or allow a check he might not be able to make otherwise), even when a player brings up that characterization specifically looking for a reward.
This also goes the other way. The entire campaign shouldn’t pivot on how a character sells themselves unless everybody agrees that’s something they want, but selling also shouldn’t exist solely for advantages. A character who talks to himself in tense situations gets a penalty to Stealth checks when he scales the castle walls. A heavy sleeper gets a penalty to initiative when the party is attacked in the middle of the night. Good characterization isn’t forgotten the moment it becomes inconvenient, and a DM should work both the good and bad traits of a PC into the game somehow. When a player gives their character a specific trait or quirk, it means they want to play that trait or quirk in-game, not declare it on a character sheet and ignore it thereafter.
Any game can be separated into “rules” and “things that are not rules”. Selling is how a player presents the latter category in a way that affects the former category. Its goal is to reconcile conflicts between the two; a player can say her fighter is cruel and heartless, but if she also has absurdly high modifiers on Persuasion and Medicine checks, how she bridges that gap says a lot about her and her character. It’s optional in that nothing in the rules requires it. But it builds characters, helps form memorable moments, increases immersion, supports story arcs, and even gives power-gaming players leverage to work their magic, and it’s explicitly recommended by the official books. It’s the “role-playing” part of the “role-playing game”, and without it D&D is just gambling and accounting.