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 actions as believable and appropriate.
But it is also important to render a character’s actions in proportion to the stimulus that initiates them.
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 Some Like It Hot, the director, Billy Wilder, was asked why he opened the film with the St. Valentine’s Day Massacre. He explained that he needed to provide the audience with a powerful reason why the two musicians, Jack Lemmon and Tony Curtis, would dress up in girls’ clothes (used to generate the many priceless moments in the film). Their need to hide their identities from the mob makes their behaviour credible.
A character’s actions will be believable if they are justifiable and proportional to the event that initiates them.