The use of "markers"
After opening with a kinetic, often hilarious slapstick action scene involving Wick's beloved Mustang, John Wick: Chapter 2 quickly establishes its noir bona fides by having our Zen hero accept a mission because of a "marker" that he owes to someone. That debt belongs to Santino D'Antonio (played by Riccardo Scamarcio), the Italian gangster who'd helped Wick leave behind a life of crime in favor of marital bliss. (Wick's wife, Helen, died of an unspecified illness prior to the events of the first movie.) What mission does Wick get sent on? He must head to Rome and kill D'Antonio's sister -- and, if he refuses, he'll be killed.
The idea of a "marker" as a way to keep track of a debt is a staple of crime fiction, mafia sagas, and hitman movies. It just sounds cool and mysterious, conjuring an idea of an elaborate hidden shadow economy based around fancy IOU notes passed between disreputable men and women. Like any moral system shared between killers -- whether it's in Westerns or Samurai films -- the "marker" is a codified custom of a lawless world, a way to create order amidst a seemingly anarchic system. And, of course, it's meant to be broken.
The fun of John Wick: Chapter 2 is in watching the filmmakers find new ways to provide clever twists on old tropes. The movie's writer Derek Kolstad, who also penned the script for the first film, originally introduced the concept of the "marker" as a way to simplify the story's plot. "It was like a favor," explained director Chad Stahelski in an interview with SlashFilm. "It’s a bond, it’s a check you write with your life. It was taken in a different way. [Kolstad] wanted to use it in a different way, and we’re like, we love that idea, there’s something mythical that it’s a talisman. There’s something cool about that. You trade your life for a favor."