In The Firm, a 1993 legal thriller with Tom Cruise, there’s a great example of a storytelling technique called Chekhov’s Gun.
Named after Anton Chekhov, who advised in letters to young playwrights that“One must never place a loaded rifle on the stage if it isn't going to go off. It's wrong to make promises you don't mean to keep.” The term explains that every element in a story must be necessary, and if it’s not, it should be eliminated from the scene entirely.
Three times throughout the movie, while Tom Cruise’s character is making his way into his office, there is cotton truck blocking an alleyway. For some odd reason, you hear the voices of the security guard and the driver of the truck going at it louder than you hear other extras on the street. A reference is also made by the security guard that he’s tired of having this conversation with the driver every day, hinting to the viewer that the truck blocking the alley is a regular occurrence.
By the end of the movie, the cotton truck does not seem to have any importance. But true to the essence of Chekhov’s gun, it’s there for a reason.
When Tom Cruise eventually has to make a daring escape from the law firm’s building, he uses the cotton truck to catch his fall after jumping from a third-story window.
If that truck wasn’t ever there before in the movie, it would be an occurrence of deus ex machina, or what happens when a plot is resolved in a very unlikely way.
But knowing the cotton truck was there in both Acts 1 and 2 of the movie, the viewer doesn’t feel like they’re being cheated, and the question every viewer is asking – “Why is the cotton truck parked there every day and why do we hear them yell so much about it,” – is finally resolved.