This is one I mentioned earlier, as part of the Chemistry between the two sisters. A Shared Bit, as I define it, is a recurring routine. It can be something physical, like a special handshake. Or, it could be verbal, like two friends who always try to show they have a bigger problem than the other one. A bit can be comedic, but doesn't have to be.
Here's an example of a verbal Shared Bit: There are two friends, and one is always announcing the subtext (the buried feelings) of the other. This would be their Shared Bit.
Take a look at another case study: In this game, you play a paramedic. You walk into the employee lounge at the hospital. There you see Jim and Steven, who are frequently seen together.
PLAYER (to Jim): You're eating my sandwich. STEVEN: He's upset, he brought in a stiff. Baloney calms him down.
And then later in the game, you find Jim and Steven in your ambulance, about to pull out of the parking lot. Jim's at the wheel; Steven's riding shotgun. You walk up to them.
PLAYER: (to Jim): Get out of my ambulance. STEVEN: Your ambulance is newer. It helps his self-esteem.
This Shared Bit between Jim and Steven shows Chemistry between them.
Shared Bits don't prompt player action and they don't advance the plot.
Their purpose is, like all the techniques in this chapter, to make us believe that two NPCs belong together.