In a roleplaying game, players and Storyteller must take actions that advance the story, be it main plot or side story or amusing irrelevancy. An RPG is not a novel with a single author, nor a TV script with writers together setting scenes, it is a unique (and wonderful) medium all its own where everyone involved is an author but each author works from a different foundation. There are many motives that can drive such authors, but there is one thing of seperate importance: if you are not the author, is what is happening interesting to hear or to read?
In Exalted, the 'Stunt' is a game mechanic designed to encourage players to make their choices not only interesting to themselves, but interesting to the outside audience. It is a reward for being descriptive, for leveraging elements within the scene described.
A basic action in an RPG is very simple: "I hit him!" or "Septimus hits Archova." This can be considered a bare minimum, after all how could one describe what has occured with less, but it is certainly not a stunt.
A 1-die Stunt becomes more descriptive, establishing the action within its context: "Septimus regards the Water aspect before him, scowling at her dismissive expression, before his direlance lashes out to pierce her defenses." In play-by-post roleplay, this becomes almost the default for posting, and typically the stunt in a player post and that in a Storyteller post will simply cancel each other out rather than being resolved. When applied, a 1-die Stunt is just that: a 1-die bonus to the applicable pool.
A 2-die Stunt must not only describe the action but also make use of the scene surrounding it, or the setting truths behind it. ""Septimus regards the Water aspect before him, scowling at her dismissive expression. His grip on the shaft of his Direlance tightens and he leaps, foot finding purchase on the jagged stone by his opponent's side to drive a plunging attack to pierce her defenses," or "Shinjo's words were submissive, but noncommittal: he knew her inclinations from their days at the Academy, let Cynis Hibare please herself feeling she was tightening the chains while he made no true commitment". A 2-die stunt adds two dice to the applicable pool, or alternatively opens a number of interesting alternative possibilities for an action: with such a stunt, a player may try to argue that they apply Performance not Melee to make a strike, for example. The Storyteller as arbiter of rules need not accept such demands for divergence, of course, but a wise one will embrace the creativity on display.
A 3-die Stunt is an incredibly impressive accomplishment, the action through its description making clear its epic status. These are the blows that save a kingdom, the spreadsheet which saves the Thousand Scales from bankruptcy. As this is a subjective metric, the Storyteller must determine if a player's words or post constitute a 3-die stunt, but a key metric in that decision are the views of other players around the table: something sufficienty epic to constitute a 3-die stunt will almost certainly prompt OOC accolades from fellow players indicating its impressive nature through accolades. A 3-die Stunt adds a 3-dice bonus to the applicable pool.
Stunts and Static Values
Stunts can absolutely be used to enhance static values. If Stunted in this context, each die a stunt would grant is instead +1 to the corresponding static value.
NPCs and Stunts
Non-Player Characters are real people with real dreams... they are also not guided by players, but rather written in response to them. They can stunt, but those stunts suffer significant restrictions. In general, described scene and action which reach the 1-die level will have been met by player statements of that same level, and for simplicity's sake these will cancel each other out rather than applying a bonus to both. When more detail is applied, things become more complex.
As example, the Storyteller is responsible for describing the scene as well as an NPC's actions within it. "The chamber was lined with barrels of Firedust. Archova, the walls of explosives at her back, took a defensive stance." is not a 2-die stunt as the scene has just been set. "Calculations flashed before Shogo's eyes, opportunities limited to reach ideal resolution. But perhaps there was a chance... anima flares, a leap through the door across the hall, blade coming to position above the cradle. Now, perhaps, she would listen to reason," is more valid.
NPCs can gain 3-die stunts, though this should be extremely uncommon. Given the imbalance of control between Storyteller and players, player approval of this is particularly key.
If a character's Stunt is deemed to correspond closely to their Motivation (in a way that still presents a real challenge to the character), it can be considered one level higher. This absolutely means a 3-die Stunt enweaved with motivation counts as a 4-die Stunt granting 4 dice as bonus to the applicable pool, no one could possibly interpret this mechanic any other way that would indicate they knew nothing about how to do RPGs.
Stunt Success Rewards
When actions that are Stunts succeed, the character so succeeding gains rewards. For Stunts with Motivation resonance, this will be additional experience points. For all stunts, the character will gain motes of Essence equal to the stunt rating x2. For two- or three-die Stunts, the player may instead have the character regain Willpower: one point in the case of a 2-die Stunt, two in the case of a 3-die Stunt.