Step 1 - Continue the main story until you unlock the attic.
Complete consecutive stages of the main story, during which you have to visit various parts of the block of flats, investigate crime scenes, and hack into other people's minds. In the last case, pay close attention when hacking Amir Nowak's mind, as the Drug Trail achievement is involved.
Continue the game until you get the objective to trace the murderer. The blood trail will lead to the attic showed in the picture above - do not go there, until you complete all the cases in the main part of the building because after entering the attic you will not be able to go back.
Step 2 - Complete side missions before you go to the attic.
Before you go to the attic, you have to deal with all the optional activities, which unlock particular trophies. It is a good idea to do this at this precise stage of the game, as the locations that were inaccessible at the beginning are now unlocked (e.g. tattoo parlor and the basement).
Most importantly, unlock the following achievements: KPD! Open Up!, Obsessive, A Work in Progress, Lord of the Harvest, and Body Snatcher.
Additionally, you have to make sufficient progress in order to unlock the following trophies:
- Find 46 out of 69 Nanophage Cards for the Pandemic trophy.
- Locate 9 out of 10 computers with a mini-game for the Noble Sword achievement (the last computer is at the murderer's hideout).
- Find 2 out of 4 roses for the By Any Other Name trophy.
- Find 3 out of 4 recordings for The Root of All Evil achievement.
Step 3 - Continue the main story until the moment when you have to decide about the ending.
Continue the main story and look around for more items necessary to unlock the trophies mentioned in step 2 (Nanophage Cards, roses, the last computer, and the last radio with a recording). Additionally, you have to pay attention to a yet another trophy called Voxel Runner, which regards one of the hacking sequences (it is optional and possible to miss).
Continue until you reach the location in cyberspace shown in the picture above. There, you meet Adam for the last time and you have to decide on the ending of the game.
Step 4 - Seeing both endings of the game
If you are playing Observer on PC Windows or PS4 it is highly recommended to create a backup copy of the save files before making the decision on the game's ending. This will allow you to see both endings one after another. In the PC version of the game, the save files are located in a hidden folder: C:\Users\username\AppData\Local\TheObserver. On PS4, you can copy your save files onto a USB drive.
The endings are linked with the Prodigal Son and Death in the Family trophies. Choose either one of them. If you have a backup copy, turn off the game and replace the existing save files with the ones from the backup. Upon resuming the game you will be able to choose the second ending and unlock the second achievement. Should you not use the option with the backup save files, you have no other choice (Xbox One version of the game) but to replay the whole game (annotation - all the secret items such as Nanophage Cards have to be collected from the start).