This is a script I wrote for @tibfox's request.

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Characters:

The THIEF. He wears an ALL BLACK OUTFIT including a BALACLAVA and has a WATCH, a NOTEBOOK, a sleek BACKPACK.

The TENANT. He wears a SUIT outside or a DRESSING GOWN inside the apartment.

The SECURITY GUARD. She wears a UNIFORM.

Locations:

The APARTMENT. The apartment is RICH, SPACIOUS but CLUTTERED. There should be a BALCONY, a BEDROOM, a SAFE with a COMBINATION LOCK, a WARDROBE, and a WEIRD ALTAR.

The ENTRANCE of the flat. Pass through here to get to the APARTMENT using the LIFT. There is a CLOCK, a DESK, and at the desk sits the SECURITY GUARD.

The DESERTED LANDFILL.

Props:

The THIEF'S NOTEBOOK is a Moleskine-like black notebook. It contains hand-drawn schematics of the APARTMENT and a list of the Thief's planned actions, all timed to the second. Also a drawing of the SAFE and pages and pages of numbers and possible combinations.

The WEIRD ALTAR is a small set of Celtic-looking props, like stones carved with runes and patterns and a brass plate for offerings. It is placed on a piece of furniture out of the way. Close to the altar but unseen is the STRANGE KNIFE.

The ANCIENT JOURNAL is written in code. It contains occult sketches, dangerous diagrams, forbidden incantations. One page depicts the WEIRD ALTAR and includes annotations about its uncanny properties.

The STRANGE KNIFE is acutely pointy.

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THE CHANGELING

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Ext. Night.

On a rooftop. The THIEF opens his notebook and skims through the pages detailing the HEIST. There's enough lighting to read some of the contents. The Thief checks his watch, it's 8:00.

The Entrance. Night.

Focus on the clock on the wall. The time is 8:00. The TENANT enters from outside. The SECURITY GUARD barely lifts her face from a magazine.

TENANT
Good evening, Mrs. Roberts.

SECURITY GUARD
Evenin'.

The Tenant enters the lift.

Ext. Apartment, the Balcony. Night.

The Thief rappels down from above. He lands on the balcony, retrieves his equipment, and crouches behind a convenient obstacle. Right on cue, the Tenant arrives from inside the apartment. He walks the length of the balcony, looking everywhere except where the Thief is crouching. Seizing an opportune moment, the Thief sneaks behind him and into the apartment, undetected.

Int. Apartment. Night.

We follow the Thief as he moves from a hiding place to the next. The Tenant comes back in and out of sight and goes about his business, never once noticing the Thief, despite some near-misses. It feels like the Thief anticipates where the Tenant will go next and moves out of the way just in time.
The Thief reaches the safe in the bedroom. He takes out his notebook and studies it. As the Thief touches the safe, the wind chimes in the hallway ring. The Thief climbs into the wardrobe but looks out through the narrow slit left open by the wardrobe door.
We see an agitated Tenant leading the Security Guard into the bedroom. The Security Guard has drawn her gun. The Tenant is biting his lip, while the Guard searches under the bed, behind the curtains, in every possible nook--except the wardrobe.

SECURITY GUARD
Sorry, sir. There's no one here.

TENANT
(agitated) I'm telling you, I know somebody got in. Check again!

The Security Guard goes through the motions while the Tenant fumes. She even comes close to the wardrobe, but strangely does not look inside.

SECURITY GUARD
Sorry, again, sir. It must have been your imagination.

TENANT
(resigned) Useless...

SECURITY GUARD
Just call me again if anything happens, sir.

The pair walk out of the bedroom and their voices die out.

The Thief carefully exits the wardrobe and produces a GARROTE. He slowly creeps about the apartment. He finds the Tenant next to the WEIRD ALTAR, sucking his bloodied thumb. A delicate wisp of incense-like smoke rises from the altar's plate.
Swiftly and expertly, the Thief approaches from the Tenant's blind spot, places the garrote around the Tenant's neck, turns around, and lifts the Tenant on his back, strangling him. The Tenant struggles and reaches back, but the most he can do is remove the Thief's balaclava. We see the Thief's impassive face for the first time and notice it's identical to the Tenant's, though their visages are never shown at the same time. The Tenant dies.

Int. Apartment. Day.

Sunlight reaches the bedroom. We find the new 'TENANT' wearing the dressing gown. He checks his watch, it's 7:30. He's spent the night working on the safe; we see a stethoscope next to it, and the open notebook on the bed, new combinations present, most of the numbers crossed out.

The Entrance. Day.

Focus on the clock on the wall, it shows 8:00. The TENANT exits the lift wearing the suit and carrying a heavy bag. The SECURITY GUARD barely lifts her face from a magazine.

TENANT
Good morning, Mrs. Roberts.

SECURITY GUARD
Mornin'.

The Tenant exits the flat.

The Deserted Landfill. Day.

The TENANT drops the heavy bag on the ground. He unzips it, briefly revealing a clean bone saw, an all-black outfit, and human mortal remains, neatly reduced to medium-sized pieces packaged with plastic and tape. The bag rolls down a slope and the Tenant pushes trash to cover it.

The Entrance. Night.

Focus on the clock on the wall. The time is 8:00. The TENANT enters from outside. The SECURITY GUARD barely lifts her face from a magazine.

TENANT
Good evening, Mrs. Roberts.

SECURITY GUARD
Evenin'.

The Tenant enters the lift.

Int. Apartment. Night.

The TENANT is back in the bedroom, again wearing the dressing gown, listening to the safe using the stethoscope. Finally, he finds the codes and opens the safe. Inside we see LOOT and an ANCIENT JOURNAL. He ignores the valuables and picks the journal. The Tenant stops at the drawing of the WEIRD ALTAR and follows the annotations with his finger.

Now distraught, the Tenant puts the journal back in the safe and locks it in. He hurries to the balcony, inspecting every spot, especially the one where the Thief hid the previous night. Nothing's there. He returns inside and looks around; we confirm he is alone in the apartment.

The wind chimes in the hallway ring, startling him.

The Tenant comes to the bedroom and dramatically opens the wardrobe, but it's empty. He moves through the apartment, at times suddenly turning, to surprise anyone that might be skulking behind him.

As he walks by the WEIRD ALTAR, he does that again. And standing right in front of him is the new 'THIEF', holding the STRANGE KNIFE. The Tenant is stabbed repeatedly and savagely through the chest. We see some of the blood spatter ending on the altar's plate, possibly activating it. The Thief removes his balaclava, again revealing he looks identical to the dead Tenant. Again, their faces are never shown in the same shot.

The Entrance. Day.

Focus on the clock on the wall, it shows 8:00. The TENANT exits the lift pulling a big trolley. The SECURITY GUARD barely lifts her face from a magazine.

TENANT
Good morning, Mrs. Roberts.

SECURITY GUARD
Mornin'.

The Tenant exits the flat.

Int. Night.

As credits roll, we see the WEIRD ALTAR and the delicate wisp of smoke rising from the altar's plate. The scene cuts to an abandoned trolley. Something moves inside.