Riptide allows the other characters to show up at safe areas in single player. The normally calm and stoic Xian has an Idle Animation of fooling around with a knife, even accidentally cutting herself on it, then keep playing with it.
While the other survivors are not afraid to loot a location John is rather...eloquent about it.
Dr. Kessler: "There now, that wasn't so bad. If I had a little sweet I would give you one."
If you play as Morgan his reply is thus:
Oh Jesus Christ on a pony…
One of the sidequests "Superhero" involves a man named Eddie who tasks the player with retrieving his cape, weapon and hero boots (a dirty blanket, a "super" fish, and some old shoes) and delivering them to him so that he may save Henderson by becoming the Zombinator. When the player returns to him he then has them accompany him on his "epic quest" fighting the undead. He hasn't even walked all the way down the street when a group of Infected show up and attack. The player easily kills them but in the chaos the "Zombinator" took off and ran all the way back to his safe area. Upon returning he decides that the player is probably better off fighting the zombies alone and hands over his reward. Really, the whole mission is almost completely at odds with the tone of the rest of the game... but it sure is funny. Gee thanks Ed.
The characters had become considerably more vocal, as well as a lot snarkier, as an early scene demonstrates on the ship where you investigate the galley.