This revelation of someone being other than they appear to be can be done through special effects, but I am especially impressed when it's conveyed through story and acting (i.e., a shift in facial expression) alone.
In The Broken (2008), Gina (Lena Heady) suffers a car crash. Physically okay, she is now plagued with amnesia -- and a growing suspicion that she has a double (i.e., a doppelganger, though that term is not used) who is somewhere out there, following her. Why?
Gina then grows suspicious that her boyfriend, Stefan (Melvil Poupaud), is not her boyfriend. That he's been replaced by his doppleganger.
If it sounds like Invasion of the Body Snatchers, that's no accident. The Broken pays direct homage to the 1978 remake when a frightened Asian man tells Gina's brother, Daniel (Asier Newman), "That's not my wife."
The Broken's conceit is supported by a creepy atmosphere that's achieved through 1. long stretches of silence (occasionally supported by some unsettling ambient noise), and 2. extreme closeups of mundane objects.
(David Lynch has used these same techniques to great effect, and The Broken continues borrowing from Lynch through to its penultimate scene, with a musical score that's reminiscent of the one at the end of Mulholland Drive.)
The Broken packs its greatest emotional punch at films end. Actually, two punches.
The First Punch is in the penultimate scene, in the form of a Big Revelation. Gina's long hunt for her doppelganger ends when she discovers her own dead body in her apartment, whereupon Gina's amnesia lifts and she remembers that She is the doppelganger!
I saw this revelation coming about 15 minutes before it did. It's not too original a plot twist. Many films have protagonists who discover at the end that they're really the villain (Total Recall, Thr3e, Number 23), or really the good guy (Murder by Night), or a ghost (The Sixth Sense, The Others) or dead (Jacob's Ladder). Ideally, the audience is likewise surprised. Having empathized with the protagonist, they emotionally share the protagonist's shock and distraught.
But it's The Broken's final scene that makes it a truly great horror film. Its Second Punch is a Personality Shift that is one of the scariest horror scenes of the past decade. It's a scare that's achieved without special effects, but through story and acting alone.
In this final scene, Gina is at work, knowing that she is a doppleganger. She exits the room to see her brother, Daniel, in the hallway. She'd earlier warned Daniel about the doppelgangers. Daniel has by now seen the personality transformation in his fiancée.
We'd last seen Gina as a sympathetic character. A woman distraught at learning that, before her amnesia, she had been a murderous doppelganger.
She approaches Daniel, a blank look on her face. Is she still the sympathetic doppelganger with a conscience? We can't tell from her expression.
Daniel stares at her. Saying nothing. Wondering if she's now also one of them. (She always was, though she -- and he -- didn't know it.)
An expression of hate clouds Gina's face. The same cold hate we'd seen on the other doppelgangers. The camera moves in closer to emphasize Gina's expression. She remains silent. No warm words of greeting to her brother.
Whereupon Daniel runs away in fear.
This is the Second Punch. It's the scariest scene in the film because we have grown to empathize with Gina. She had been warm and loving. The First Punch was shocking, but it didn't mean we couldn't continue sympathizing with her as a doppelganger. Bruce Willis remained sympathetic in The Sixth Sense, though he turned out to be a ghost. Arnold Schwarzenegger remained a hero in Total Recall, though he learned had been an evil government agent before his memories were removed.
Gina's emotional acceptance of her villainy is the real terror of The Broken. Not the initial terror of her being stalked by doppelgangers. Not the second terror of discovering that she's one of them. But the final/third terror of her embracing her dark side -- of her personality transforming into entirely new person.
Some monsters resist their dark side. Gina didn't. She became evil before our eyes. A transformation achieved largely through Headey's performance.
Daniel's discovery of Gina's personality shift evokes Invasion of the Body Snatchers's scene where Nancy's discovers that Matthew has become a pod person. In both films, a frightened mortal approaches a trusted friend, only to have that friend's face reveal that they are no longer the same. But Donald Sutherland's monster -- although suggested by his performance -- also benefited from the sound effects emanating from his mouth. Not so with Lena Heady.
Lost Souls has a character shift that's similar to Gina's -- a scary transformation implied largely through acting. At the film's end, Peter (Ben Chaplin) is about to become the Antichrist on a specific time. (The date and time of his birth, 33 years ago.) He urges Maya (Winona Ryder) to shoot him after the transformation.
The time arrives and Peter insists that the transformation didn't occur. He begs Maya to put down the gun. Maya is confused. Should she put down the gun? Then the car's clock blinks 666 -- indicating that now the transformation has happened.
Ben Chaplin's expression changes, indicating that now Peter is the Antichrist.
Shifts in facial expression, especially when a trusted person is suddenly revealed to be evil, are an effective way of scaring audiences. Many examples exist. To cite just one more, consider Ray Wise's changes of facial expression to suggest Bob's possession of Leland in Twin Peaks.
For more about acting techniques in horror films, see Horror Film Aesthetics: Creating the Visual Language of Fear. This blog represents a continuing discussion of my views on horror, picking up from where the book left off.