BioShock 2 opens with a flashback.  You are escorting a young Eleanor about her Little Sister tasks when Sophia Lamb stops you.  Using what appears to be hypnotically-embedded commands similar to the one used on Jack, Lamb orders you to take your helmet off and shoot yourself in the head.

It is a sufficiently shocking opening, and instantly recalls the closing act of BioShock 1.  Especially the themes of control and the player/avatar relationship.

As the game continues (ten years later), you are given the chance to kill or spare some key members of your's or the Lamb's past.  There are no overt "Yes/No" prompts here as with the Little Sisters.  If you choose to strike down these unarmed people, you do so seamlessly within the confines of the game.  There are some immediate consequences, but the primary effect is not felt until late in the game.

It is revealed that Delta was once a deep-sea diver who stumbled upon Rapture by accident, and was welcomed briefly... until the first outsider became the first Big Daddy.  It is obvious that Delta does not remember this former self, and the player learns little of him.  As for Delta, the only non-player-driven action comes in a late first-person-cutscene as Delta slams his fist on a window in frustration and helplessness as another character attacks Eleanor.

The pivotal plot moment comes when Eleanor reveals that she has been watching Delta, and learning.  Despite ten years of Lamb's indoctrination, she was never able to shake the pure love and adoration imparted by her Little Sister/Big Daddy bond with Delta, and allowed his actions over the events of the game to become her moral compass.  It is Eleanor, not Delta, who decides whether to kill or spare her mother, and Delta's actions provide the precedent by which she will base her life, as she takes her brilliant intellect and mastery of ADAM/Plasmids into the world above.

While it is a quite different experience from building, say, a family in The Sims, or sewing the seeds of an in-game garden and returning to see it fluorish, it is similarly gratifying to see the actions of your playthrough echoed in the final cinematics.  
In the closing moments of the game, Eleanor transfers your dying consciousness into hers (ADAM becomes the "a-wizard-did-it style force in the BioShock canon towards the end of the game), and you even see this as a shift in the camera perspective.  You go from seeing Eleanor perched over you to her/your hand pulling a needle out of Delta's lifeless body.

It is a wonderfully symbolic transformation of control and cause/effect relationships, and works between gamer and game character, but I found myself wondering...

...how the heck does it work in-game?

As mentioned, Delta was once an explorer.  While explorer's are stereotypically considered to be idealistic, indefatiguable and brave, little is explicitly revealed about Delta's former personality.  Are we supposed to assume that this personality has returned to the fore when Delta wakes up?  Are we, the gamer, supposed to ...assume... this identity?

Something else to consider: In Rapture, Big Daddy's are driven by a strongly encoded urge to protect Little Sisters.  They do so without regard for themselves, and it is strongly implied in BioShock 2 that this bond was modeled after natural father/daughter bonds in order to improve its effectiveness.  Big Daddies who lost their Little Sisters were reported to be comotose or driven to uncontrollable rage.

When I play single-player games, I am a role-player.  It does not matter if the game tracks stats or uses dice rolls to calculate damage.  If I can look in a mirror in-game and see a character looking back at me, I try to PLAY that character.

Given that Delta's former identity is not revealed until mid-game, I set out playing BioShock 2 as I would expect a true smarter-than-average Big Daddy to behave.  Splicers were not trying to kill me, they were trying to keep me from reaching Eleanor.  Little Sisters I stumbled upon were not sources of ADAM to be collected, they were triggers for my Big Daddy programming and I was forced to help them.  When the game let me, I made leaps of logic that let me keep playing the character.

...When I spared the certain characters, I did not do it out of a sense of mercy or justice, I simply ignored them, just as the other Big Daddies ignored most of Rapture.

But something odd happened.  While I thought this was a natural course of action, the characters around me were shocked.  My inaction became a message to them.  One character saw that I was not the mindless demon that Lamb had told her about, and concluded that Lamb was not to be trusted.  Another was surprised, and attributed my actions to a previously unobserved individuality.  He would later admit to his intended betrayal, and vow to risk his life on my behalf.

I stumbled at this point.  Both conclusions ran against the way I was playing Delta.  Delta was, after all, just another Big Daddy.  Big Daddy's don't go out of their way to harm others, they simply seek to protect the one being in their life who is beloved above all else.  Later, I reasoned that these characters were projecting upon me.  They felt guilt, either for their past actions or for their future intent.

Thus, as I watched Delta join Eleanor for the final time, I was able to maintain the connection between gamer, character, and avatar.  She projected onto Delta the counterbalance she needed against her mother's warped societal ideals.

The niggling doubt, though, comes when I considered what could have been.  Gamers can be selfish, vindictive pricks when they want to be.  While I could never bring myself to harvest a Little Sister (I still don't know what the animation looks like -- 2K Boston devs should swell with pride at that), I am sure plenty of gamers were not so queasy about it.

How would a murderous Delta figure into Rapture?  Would he be a chemically-imbalanced psychopath, irreparably damaged by the gunshot he received to the head at the beginning of the game?  Would a resurging deep-sea diver personality turn out to be a selfish survivor, putting his needs before others at all cost?  The former possibility would only have player-actions to serve as evidence.  The latter is less of a stretch, but there is still little in-game evidence to support it.

Taking this torch and running with it, what other medium could get away with casting the protagonist as a hulking serial killer who regularly drinks the ...fluids... of innocent little girls, on camera?  Could anyone get away with telling such a story, if they did not have the audience literally giving them cues (in the form of player action) to continue?  What do you think?