What matters here is that the walking mechanic allows a tiny amount of novelty that has not been designed in advance. No two strolls across that room need be the same. This same principle can also apply to shooting zombie hordes and to countless other mechanics based on repeatable, iterative action -- shooting, punching, jumping, walking, driving, swimming, etc. Any mechanic that offers the possibility of a unique outcome -- however trivial -- will likely allow a player true Agency. And the more Agency mechanics a game possesses, piled one atop the next, the more complicated the possible outcomes become.
A second and related feature of Agency mechanics is their a "two-stage life cycle": Designers must first carefully craft and balance an Agency mechanic before letting them loose. However, once released into the wild, Agency mechanics must rely on players to give them an additional layer of meaning, to "bring them to life."
In other words, gamers themselves become craftsmen of a second order, charged with finding unique ways to use these systems to their advantage. If a game's mechanics are robust enough, certain unique styles of play will emerge, as is the case with older games like Chess and Go. Today, newer games like StarCraft, Far Cry 2 and 3, DayZ, The Sims, and even Super Mario Bros. allow their players a related form of creative expression.
Examples A and C, however, are examples of Destiny mechanics; forms of interactivity that are set in stone and offer only one or a finite number of paths forward. Branching dialogue trees like those found in Skyrim, Deus Ex, and Monkey Island offer a form of player choice, but almost never a systemic one. The same holds for the treasure map puzzle alluded to above, as well as most graphic adventure game puzzles, for which there is only one order of operations (unless the designers are being coy).
Dialogue trees, fetch items, mazes, riddles, lock-and-key puzzles and the like are functionally linear in the sense that each choice available to the player has a predetermined output, mapped by a designer to have a specific effect in the long run. There is no underlying system, and merely the illusion of Agency: a finite number of roads have been paved, and the player's only choice is to walk down one. Viewed on a grander scale, we can see that entire branching narrative paths like those found in the Mass Effect series and the Walking Dead series offer choices, but not true Agency.
But this is not a slight against the beauty and utility of a Destiny mechanic, for what they lack in systemic flexibility they usually make up for in creativity and depth of meaning. It is difficult to imagine puzzles as unique, amusing, and narratively specific as those found in Portal, Grim Fandango, Myst, or Braid emerging from purely systemic gameplay. Such intricacies require a designer to craft and refine them. For this reason, Destiny mechanics do not require a player's creative input, only his participation.
This means that Destiny mechanics are forged almost exclusively the domain of designers, not the players. They can be sculpted, shaped, and polished for maximum challenge or emotional impact, but since there is no system to master, nor is there a repeating set of problems to solve, the player will always be something of a passenger in this particular vehicle. It would make no sense, for instance, if you heard someone claiming to be "really good at Heavy Rain" or "an expert Myst player" in the same way that others are "professional Quake players," for the simple fact that the former two are predominantly comprised of Destiny mechanics, while the latter is Agency heavy.
But from a basic design standpoint, both Agency and Destiny mechanics are perfectly viable and powerful tools for the modern developer. So it should come as no surprise to find that most modern video games are complex and often bizarre hybrids built on foundations of both kingdoms. Scan the triple-A console market and you'll find the usual suspects are perfect examples: Uncharted, Assassin's Creed, Call of Duty, and The Elder Scrolls series. Each of these titles overlay a tightly scripted set of strictly controlled narrative sequences atop a robust systemic layer. Here, Agency and Destiny mechanics sit side-by-side. Not always in harmony, but hoping for the best.
Let's look closer at the Uncharted series, a constant critical darling. Here you'll find your basic shooting, running, and swimming mechanics, all of which are truly systemic. Players decide what weapons to carry along their journey, when to fire off a few rounds, whom to shoot first and last in every encounter, and when to duck behind cover. No two firefights are ever the same, meaning the combat system in Uncharted allows true player Agency. But only within the confines of combat.
Almost everything else the player experiences as Nathan Drake -- the expertly carved navigation challenges, the environmental puzzles, the action set-pieces with clear narrative arcs -- is an experience whereby the underlying Agency mechanics have been yoked to a surface layer of Destiny constraints.
All these moments are tightly controlled, linear, and breathtakingly specific in their action -- a collapsing building, a moving train, a capsizing boat. And though the high-drama, pounding music, and incredible mise en scène of these moments gives each player the impression that he just barely made that leap away from certain death, in truth that's all he was ever going to do. Because that's what the designers required him do. That was his destiny.
If the velocity of my argument up to this point isn't clear, let me make it so: well designed Agency mechanics allow players to experiment with their own creative impulses, while Destiny mechanics are canvases for the designers' imaginations. And it is this complicated split that lies behind so many of our strained conversations about video games and their value. What happens, for instance, when a game contains only one type? Or a conflicted mix of both? And how does this affect our experience with them?