In his book, Film Scriptwriting: A Practical Manual, Dwight V. Swain offers us two principles that underpin verisimilitude in stories – justification for everything that happens in the tale and a proportional response from the character to the events that confront him.
Justification boils down to the readers and audiences believing that given a specific personality type, a character would react to a challenge, to any sort of stimulus really, precisely in the way that he does. In short, if your readers understand why your character acts in a specific way, they will experience his or her actions as believable and appropriate.
But it is also important to render a character’s actions in proportion to the stimulus that initiates them.
“Improbable action can be made probable by having it spring from the twin launchpads of justifiability and proportional response.”
Exaggerated, unmotivated behaviour, under normal circumstances, can spoil a scene. If a girl turns down a casual request for a date from a man she hardly knows and he then proceeds to burst into tears, his behavior would be considered an overreaction.
If, on the other hand, a child were to run into a room, screaming and bleeding, and her mother were to ignore her in order to finish her bridge game, we would consider her behaviour as an underreaction.
Over and under reactions are major flaws that undermine believability in stories.
In interstellar, the earth is dying. Humanity needs to find another home. Cooper, a conscientious, widowed engineer and former NASA pilot turned farmer, lives on a farm with his father-in-law, his 15-year-old son, and his 10-year-old daughter, Murphy.
After a dust storm, strange patterns appear in the dust in Murphy’s bedroom. Cooper realises the patterns were caused by gravity fluctuations that represent geographic coordinates in binary code.
Cooper follows the coordinates to a secret NASA facility headed by Professor John Brand, where he learns of the existence of a wormhole. When he is re-recruited by NASA to fly a mission through the wormhole to confirm the planet most suitable for mankind’s survival, he promises his distraught daughter that he will come back at any cost. This promise creates the motivational spine of the story. It helps Cooper’s actions to appear both justifiable and proportionate, despite the improbable nature of events in the story. It does this by balancing his duty to humanity with his unbreakable promise to his child.
Improbable character action can be rendered believable by making it justifiable and proportional to the events that initiate it.