We care how others treat us - Design lessons from The Walking Dead

Gabe Newell has said in a few different interviews that 'fun' is when the world reacts to a player's action.  So if you shoot a wall and there is no response, that is less 'fun' than if you shoot a wall and it creates decals.  By that definition, decals are 'fun.'

The Walking Dead makes a similarly subtle point about player choice in conversations.  They avoid the typical RPG content-sink of creating multiple large parallel plot branches (are you GOOD or EVIL?), but instead create relational "decals" from all your conversations.

Several clever and economical ways they did this:

We care how others treat us

Every character in Walking Dead tracks an emotional relationship score with the player.  This rarely impacts the plot, but it shifts the tone of almost every conversation, ranging from "You can be my Wingman any day" to "I hate you but we have to work together."  The result is structurally the same, but the tone shifts significantly based on your previous choices.

This ends up being astonishingly impactful.  We all want to be liked - enough that some choices between "Do the right thing" and "Do the thing that makes person X still speak nicely to you" was a legitimate ethical dilemma

Player agency does not require success

The player is presented with a major ethical dilemma - Do you murder the still-living bite victim in front of his family, or not?  Rather than writing a branching plot that supports each choice, NPCs in your party just overrule you and perform the action if you refuse.

This gives you all the emotional benefits of agency - you took an ethical stand, NPCs can remember and later reference your choice, without any of the content costs of actually branching the plot.

Bioshock Infinite has a couple great examples of this - they give you moral choices that don't really impact anything, but you still chose, and the memory of that choice can be reference by the player or NPCs later in the story.

Remembering small choices

The original Deus Ex had a great example of this - if you went in the women's restroom ,before speaking to your boss, he would chide you for it at the end of your briefing.  Again, no impact to the plot, but a strong impact to the player - The choices I am making are being observed.

Throwaway appended conversation that merely says "I appreciate when you did [...]" or "You shouldn't have done [...]" can energize the player with a sense of agency and persistence. 

In Mass Effect, an NPC showing up and saying "Hey Shepard, I remember when you did that thing in a previous game" was just as impactful as major plot or character changes based on the previous games.

In fact, I think this was Clementine's primary motivational role in the game.  Her job was to watch the player, and remember all your small choices.  The fact that your surrogate child was watching lent significant weight to all of your ethical decisions, and they reemphasized several times that even your choices you thought made in secret, weren't.  She's watching, and she remembers.