After collecting the three film reels and repairing the projector by replacing the projector bulb (covered in part 27 of the walkthrough) put the films into the projector in the order listed below. Each time, after you put one of the films, you will run down to the theater and go through the screen to enter the movie. By doing this, you will learn the movie theater keypad code.

1) The House on the Lake
Locate the house in the center, with the mailbox. Enter the house through the porch, go to the end of the hall (do not climb stairs) and into a small room, see the number "9" written in blood on the wall. Go back to where you entered the movie from, and exit the movie.

Downour Cinema Verite Side Quest First Movie

2) The Silent Children
Replace the movie reel with this one. Pass through the screen. Explore the house you end up in. At a corner, near the ladder, find the number "24" written on the wall.

Downour Cinema Verite Side Quest second Movie

3) The Secret of the Attic
In this third room, written in dust on the window, is the number "1".

Downour Cinema Verite Side Quest thirf Movie

Solution to Movie Theater Numeric Keypad Panel
The access code for the keypad is "9241", the numbers you saw inside the films.

Downour Cinema Verite Side Quest movie code

Punch it in, and the door will unlock. Be sure to retrieve any remaining film reel from the projector before going into the small film splicer room. Use the reel splicer to splice all three films together, and you get:

Spliced Reel
The Secret of the Child at the Lake

Then put the combined spliced film back in the projector. Go back into the movie and the apartment, then find the door on the hallway to your left to enter a small studio. On a air conditioner there, pick up the wooden crank.

You get:

Wooden Crank
A crank for a movie prop

Go back to the small room where you found the number 9, and take the door into the house from the second movie. Use the wooden crank on the Jack in the Box to get the key.  You receive:

A prop key from the movie

Go to the ladder on the corner and climb it. to the attic. Open a big trunk near the wall. The chest contains a Golden Gun and two pistol bullet boxes. You can only carry two guns at a time, so if you already have the Colt and the Shotgun you might have to leave one of them behind.

Go back to the movie theatre area. On the way you will be attacked by a Screamer, Brawler and a Weeping Bat. You can either fight them or escape. Leave the house and emerge through the movie screen.

You obtain the following achievement/trophy:

Cutting Room Floor

Complete the Cinema Verite Side Quest

Exit back to Chastain Heights.