The approximate difficulty level of the trophies - 5/10 - Observer is an unusual game when it comes to unlocking achievements and trophies. Completing the main story does not cause any troubles and it does not take much time to accomplish. It is much more problematic to unlock the trophies regarding locating various items and completing optional activities. An additional impediment is the fact that the game does not allow us to save the game manually. Making a mistake and e.g. continuing to the consecutive location too fast might make it impossible for a player to unlock certain achievements and force him to restart the game. You can try to create a backup save files (it is possible on PC Windows and PS4) in order to protect yourself from any mistakes.
During the game, pay attention to Pandemic and Noble Sword achievements, because the objects you have to find (Nanophage Cards, and computers with a minigame) in order to unlock these two trophies are very easy to miss. Additionally, remember about the Unbreakable trophy, which gets blocked after your character dies. What is worth mentioning, you can proceed to the game's grand finale the moment you decide to go to the attic. Make sure you have thoroughly investigated all the remaining locations before you go there. Also, before going to the attic, create a backup copy of the save files (if possible).
Approximate amount of time necessary to complete the game in 100% - Over 15 hours. Completing the main story (assuming you investigate the crime scenes thoroughly and complete the two optional cases) takes around 7-9 hours. You have to spend a lot of time to find all the secret objects, most importantly nearly 70 Nanophage Cards. You also might have some difficulties with completing the consecutive levels in the "With Fire and Sword: Spiders" minigame, as in every stage you have to carefully plan a way out of the dungeon. Additionally, the overall time necessary to complete the game might increase should you make a mistake (when your character dies or if you miss a location with an interactive object) and you will be forced to start over the game.
Number of trophies - 20 - there are 5 bronze trophies, 6 silver trophies, 8 gold trophies, and 1 platinum trophy to unlock.
Offline trophies - 20 - all the trophies can be unlocked in the single player mode.
Online trophies - 0 - Observer does not have a multiplayer mode.
Does the game's difficulty level influence the trophies? - No. The game does not feature the option to change the difficulty level.
Can the stages be repeated - No. You can freely move around the block of flats (consecutive areas are unlocked as one progresses through the game), however, some of the unique locations (e.g. memories) are impossible replay. Additionally, the game does not allow you to save the game manually. Upon entering the attic in the game's finale, you are unable to return to the main part of the building.
The minimum number of playthroughs - Two. Or one, if you use a backup copy of the save files. Observer has two possible endings, each one with a unique trophy - Prodigal Son and Death in the Family. By default, these endings force a player to complete the game two times in order to unlock one ending per one playthrough. You can, however, "cheat" the game and create a backup copy of the save files before you enter the attic, and face Adam. In case of the PC Windows version of the game, the save files are stored here: C:\Users\username\AppData\Local\TheObserver. In the PlayStation 4 version, you can copy the save files onto a USB drive. In the Xbox One version, there is no such option.