Art is a lie but a lie that tells the truth.
- Pablo Picasso
In real life, how often do we address the truth directly? Do we discuss what personal history makes us act as we do? Do we bare our feelings? Do we admit our wrongdoings? How often do we even stop to examine these things? And does not examining them usually require a stop from the movement and action of our lives?
OK, yes - as artists, most of us probably engage in far more examination of truth, history, and motive than the average person. But most people (I have come to realize from the blank stares I sometimes get in attempting to start conversations on such matters) rarely think, much less speak, about such things.
In fact, I've dated men who act like the question, "Why?" was not even in their vocabulary, and who genuinely had never in their lives thought to question their motivations.
This is vitally important to understand when you are writing a screenplay. We must know the truth behind our story, the history of our characters, their feelings and wrongdoings... but they must rarely, if ever, speak of these things directly. Instead, they inform the choices and actions our characters take.
To discuss these truths too openly will make your story ring false.
I was reminded of this during last week's episode of NCIS. The team had a man in interrogation. At first, he lied, to avoid the consequences of his actions. His ranged from tone was matter-of-fact and casual to offended and outraged. Then, after further questioning - his tone changed and became more urgent. One of the observing characters noted that he was probably telling the truth because he was not pausing before answering, his answers were not overly elaborated.
This made me think: to communicate a believable truth in my writing, I need the character to have a sense of urgency, to act without hesitation, and to show things simply without excessive elaboration. (Think action movie)
And, to the opposite effect, if I want to communicate avoidance of truth (which is sometimes the whole point of a story) I should slow down the pace, add hesitation, and elaborate at length - particularly over irrelevant details. (Think drawing room drama)
In either case, however, the characters show or avoid the truth through their behavior and the tone of the dialogue - not through specifically speaking about it. Speaking about truth quickly becomes either pedantic and preachy or like watching a video of someone's therapy session.
There is an excellent post here examining some specific cases.
I will add, finally, that there can be times in a screenplay to reverse the rule and have a character directly speak a truth that's been only implied up to that point.
One use is comedy. Highlighting an aspect of a relationship or action that the characters involved have been avoiding. "Get a room already, you two." Best done by a tertiary or minor character (so as not to disrupt the primary relationships). A good example was also this last episode of NCIS, when a someone referred to one of the characters as a sidekick. This then played out through the rest of the episode in small comedic actions as this character attempted to step out of that role...but was not addressed directly again. Some screenplays even have a "fool" character whose role is primarily to say that thing that everyone is thinking but won't address.
Another use is the turning point, especially the mid-point. When the character is at their wit's end, and cannot figure out what else to do, they might have a moment of reflection in which they examine, find, and possibly even articulate their real motivations. The "a-ha" moment that gets them moving again and gives them the strength to face the rest of the challenges the story will throw at them. Just... keep it brief. Like the soldier being interrogated, they should speak the truth simply and with urgency.
Not to say there is no place in art for lengthy internal examination. Novels are an excellent medium for this. Film, however, is an external art. Moving pictures are most engaging when they are, well, moving.