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I have an ambiguous script situation, 2 co-protagonists begin gaslighting the MC. They each have ulterior motives which they don't say. The reader is intended to infer the lies through contradictory statements, and the abrupt character change. The shift is so extreme it calls into question everything that has come before, and the MC is overwhelmed by the double-whammy. All 3 characters are on a negative arc. There's no reconciliation. It ends dark.

To clarify gaslighting refers to the 2 characters independently lying to the MC to discourage or discredit her. In my story the Hero is gaslighting for sinister reasons (to cover up that he assaulted her), the Paragon is gaslighting to scare the MC so she will get away. The gaslighting dialog has enough truth mixed in to be plausible, in some cases explaining hanging mysteries, or adding a twist that changes them. This is the emotional climax that pushes everyone to their ending state.

The Hero's downfall has been heavily foreshadowed, and the MC has been his foil and antagonist – it's why she is assaulted. I have a good idea how this plays out. They argue, and within this dialog the MC calls out his hidden agenda. Then we see the assault which he later gaslights about, so it's clear.

But almost immediately comes the Paragon's gaslighting in reaction to this assault, and this motive is much harder to show the reader.

The Paragon's gaslighting is basically victim-blaming, and the "twist" aspect suggests the Paragon has been expecting it since the beginning. It causes the MC to leave (it's important that she does). There's no apology, and they don't speak again. There is no "witness" character to observe, or a confidant to confess it to.

I originally wrote the Paragon's scene as another confrontation, with the MC saying "You are gaslighting me, blah-blah-blah", but that was too on the nose, too similar to the confrontation with the hero, and she can't be devastated if she also announces she can see through the ruse.

So I removed the explanation. The Paragon is stoic with a few cracks to show inner-turmoil/regret, but mostly relying on the reader to recognize this is out-of-character – except, since I just made the hero assault the MC, the Paragon could be a cruel manipulator in a surprise grimdark ending.

How do I show that the Paragon is "wounded" by this act of tough love? Or at least justify it as a strategic maneuver (in-character for the Paragon)? It's not important that the MC understands the motive at all. My concern is the reader.

Should I explain the reasons for gaslighting?

Forget necessary - don't pass up the opportunity for a moment of drama and poignance as you reveal the cracks in your stoic character's facade.

Because you have the scriptwriting tag, I'm less concerned about introducing an otherwise foreign POV, so put in just a touch of monologue wherever it might fit:

[MC]'s eyes filled with tears.

"You deserved this," [Paragon] repeated. "You'd been asking for it
all along. Did you expect my sympathy?"

[MC] shook her head disbelievingly, then turned and stumbled away.

[Paragon] murmured softly, "That's right, run away. And don't ever come back. Because anywhere will be better for you than here with us."

Still a bit heavy-handed, but that general technique ought to fit your described constraints.

Make each of the three, full well-rounded characters who have multiple interactions with not just each other but other people. And with distinct and believable personalities. Show how the two treat the woman and how it changes over time.

Your goal is for the reader to gradually figure out what is going on. There can be a tipping point, but the realization shouldn't be sudden. Let the reader ponder about motivations. Maybe even argue them in groups dedicated to your book (we should all be so lucky). The basic motivations are there but the full understanding should take some work.

I'd avoid ever using the term "gaslighting." But astute readers will know how to name the behaviors. Assuming that the Paragon is actually gaslighting vs victim blaming...both actions are lies but they aren't the same thing by a longshot. Telling someone she caused or deserved what happened may be a lie designed to manipulate her into action, but actual gaslighting isn't just about psychology, it's about making the victim doubt her (or his) grasp on reality.

For example, in one popular book/movie, a man gaslights his alcoholic wife. She drinks enough that she has periods she doesn't remember, or at least was woozy enough that she can believe there was a time gap from the night before. He fills in the gaps for her.

A (merely) emotionally destructive man would tell her she embarrassed him and herself. That nobody likes being around her. That she shouldn't go to the next outing, that no one wants her there.

A gaslighter, as this man is, tells stories of how she yelled at the host and flung plates of food at the walls in front of dozens of horrified guests. Things that never happened, not even in a lesser way. This and other stories like it are designed to make her not trust herself when she drinks (because she might harm someone) and to destroy any potential support she may have. It makes her doubt every memory she's ever had, because she can never seem to remember when she's been violent, and her own sanity.

So, no, don't explain the reasons for the gaslighting. Make them available to an invested reader.

I believe you have to explain it, somehow. There is nothing worse than an unexplained major action that determines the outcome of the story, especially when the outcome is sad. It seems arbitrary and fickle. And no, you can't rely on the audience to be thinking and rationally analyzing clues. You have to slap them in the face with dialogue, or (better) give them a visceral moment of realization.

Such a moment can be just insight into the character-trait driving the gaslighting.

This doesn't have to be dialogue, it can be body language or the character looking at a picture, or making an indirect comment to another character.

For example, your "Paragon" can "crack" and feel terrible or shed a tear at the MC's despair as she gives up. A colleague of the Paragon (or smartest person in the room) sees her and says, sotto voce, "You're saving her life. Hang on to that." The Paragon wipes her eyes and responds, "I don't know what you're talking about." Colleague nods.

The Hero, an opposite reaction. The MC is forced to leave, when he gets that information, he says "Aw. That's terrible." Then when unwatched, smirks and whispers, "Stupid bitch."

You don't need much, but IMO nothing is not an option.