Unlike previous entries in the Deus Ex franchise, Deus Ex: Human Revolution has a clearly characterized protagonist. Except for his extremely dry sense of humor, J.C. Denton was essentially a blank slate for the player, and Alex Denton had even fewer set characteristics. Adam Jensen, on the other hand, comes into his game with a long, involved backstory and several pre-existing relationships. While J.C. gets a reminder in his first mission that UNATCO operatives are police, not soldiers, Jensen has a past that defines him as a policeman, including a refusal to use deadly force against a teenaged boy. Human Revolution goes beyond this backstory, however, using a variety of rules and situations to actively characterize Jensen as a pacifist policeman throughout the game.
Like most role-playing games, Human Revolution has two modes of character advancement. Jensen's actions earn him experience towards unlocking upgrades to his augmentations, and also allow him to find resources that allow him to restock and upgrade weapons. The experience system openly slants towards a pacifist playthrough. Taking an enemy down by any means gives 10 points of experience, but knocking him out earns an additional 20 points as a "merciful soul" bonus. The nonviolent approach therefore triples the experience earned from eliminating enemies. The large number of enemy soldiers in most areas of the game means that the additional experience earned from nonlethal play can be substantial.
Because unconscious enemies can generally be looted just as easily as dead ones, this comes at no cost in terms of resource advancement. The non-lethal approach makes stealth easier, however, and a player on a pacifist run will need less inventory space and use fewer resources than one who's out for blood. As a result, non-violent players can use the extra inventory space to carry more healing items and food, or allocate the experience that would have gone to expanding the inventory towards more useful enhancements. This has its greatest effect in the early game, when credits and upgrades are at a premium.
Other systems reinforce this preference. In Human Revolution people die loudly but fall unconscious silently. Murdering guards therefore has a tendency to alert their comrades, which can become a serious problem in heavily-patrolled areas as the alarmed state cascades into a situation where open combat becomes inevitable. Unlike other Deus Ex games, enemies don't notice that they've been shot with a dart, so a non-lethal approach isn't more likely to give away Jensen's position. In this respect, the nonlethal approach has several tactical advantages, particularly early in the game when Jensen is rather fragile. The slant isn't total, because unconscious enemies can easily be roused by their allies to cause trouble again, but in this respect the challenge isn't much greater than what is needed to preserve stealth while killing everyone.
The experience and resource systems also encourage the player, as Jensen, to limit his targets. Attacking a civilian provides no experience, and no matter how well-heeled they are, civilians rarely have any resources (in the form of credits, healing, or knowledge) worth mentioning. Civilians do have a habit of panicking and running around, however, which may alert nearby guards. As a result, the rules encourage the player to behave as the character would, and keep his aim turned on those who pose a serious threat.
Meta-game goals also push the player to behave in a particular way. The "Pacifist" achievement can only be earned by playing through the whole game without killing anyone (or directly causing robots or turrets to kill anyone). Even though this goal is completely external to the game, it pushes the player to approach Human Revolution‘s infiltration problems in a particular way, resulting in non-lethal action by the protagonist.
Human Revolution also advances the characterization of Adam Jensen as a policeman by involving him in police work. Two sidequests in the game tie Jensen up in police business: the internal affairs investigation of O'Malley in the early chapters, and the bomb threat in the second visit to Detroit. Each of these quests requires Jensen to do some modest investigating, for instance casing O'Malley's apartment or "searching" the Metro station. In addition, both involve a specific request for nonlethal action in subduing particular suspects. The somewhat shady methods asked for in one of these quests might be seen as providing some insight into what kind of policeman Jensen was, if the Dirty Harry drawl wasn't enough of a hint.
Unfortunately, the game comes across as somewhat brittle when the player pushes beyond the boundaries of the Jensen character envisioned by its mechanics. A few newspaper headlines and offhand comments aside, characters who know of Jensen's deeds don't seem to have any reaction when he goes off script by, say, murdering dozens of policemen or every single person in the Tai Yong Medical building. Even the final voiceover will speak of Jensen making moral choices "more often than not" despite the trail of dead civilians. This surely saves on the cost of writing and recording dialogue, but it comes at the cost of making the player's choices seem not to matter.
Western RPGs typically give the player significant freedom to define the main character, and in this respect, Deus Ex: Human Revolution is no different. Any player is completely free to ignore the sidequests, or the experience bonus from non-lethal play. I did this on my first run through the game, where I felt it was more true to Adam's character as I envisioned him to kill the mercenaries who had attacked Sarif. Active characterization of this kind is not about imposing the developer's idea of the character on the player. Rather, Human Revolution uses active characterization techniques to reinforce a particular method of play. While this allows it to admirably reconcile the freedom of a western RPG with the desire to create a story around a defined character, Human Revolution doesn't quite make enough allowances for those who choose to ignore the message.