You have seven encounters with Marcy throughout the game, with a possibility of taking eighteen photos in total. At least ten photos are required for the best ending (i.e. the third one). Note that you do not need to be in night vision mode for shots in the dark, as long as you are pointing the camera in the right direction. Be sure to take multiple shots in each of the encounters listed below.
1) Appears briefly in the doorway to the kitchen after you ring the bell for the fifth time. (2 shots)
2) Appears briefly to the left of the first floor hall after the spawning of the mannequins. (2 shots)
3) Can be tracked by using the camera system found on the second floor. (6 shots)
4) Jumps on you after you check the bathtub on the second floor. (2 shots)
5) Can be seen between the planks roaming the basement. (2 shots)
6) Appears briefly in a corner after you turn on the searchlight in the basement. (2 shots)
7) Quickly crosses from left to right as you leave the basement. (2 shots)

The first gold bar can be found on the bookshelf located in the bedroom on the second floor.
The second bar can be found lying in a corner of the basement.
The third and the final one is to found in one of the drawers of the cabinet located in the main hall of the first floor. Note that the said drawer only unlocks after taking at least ten photos of Marcy.
After collecting all the gold bars, insert them into the respective slots of the red box found in the side room of the office on the first floor and get the pully handle within.
Use the pully handle (that you get from the red box) on the locked door in the basement. A hint is hung on the wall for the four-digit code that is required to unlock the safe within. If you can not figure it out, the code is 3179. The sixth photo which is required for the third ending can be found in this safe.
In order of appearance, locations of the photos are as follows:
1) First floor, on the kitchen counter.
2) Second floor, on the handrail.
3) Second floor, on the table in the room with the camera system.
4) First floor, hung on the neck of the mannequin that appears in the kitchen.
5) Basement, on the floor next to a mannequin.
6) Basement, in the code-locked safe.

After ringing the bell for the first time, simply return to and ride the elevator you come in for the first (neutral) ending. Do it fast enough to get the achievement for completing the game under two minutes.
To get the second (bad) ending, complete the game without collecting all the photos.
Third (good) ending requires you to collect all the photos and place them on the board which can be found in the kitchen on the first floor. After that, remove the planks covering one of the doors on the second floor hallway, read the note hung on one of the walls of the room behind and return to the kitchen to find Marcy waiting for you.

To access the easter egg which is mentioned at the stats screen that comes up after a successful run, follow these steps.