Young Alex costume - At the "Press Start" screen, press Up(2), Down(2), Left, Right, Left, Right, B to unlock the Young Alex costume. If you entered the code correctly, you will hear the sound of a page turning.

Alternate endings - Successfully complete the indicated task to view the corresponding ending:

Bogeyman: Do not forgive Alex's father, do not kill Alex's mother, and do not save Wheeler.
Drowning: Do not forgive Alex's father, and kill Alex's mother.
Good: Forgive Alex's father, and kill Alex's mother.
Hospital: Forgive Alex's father, and do not kill Alex's mother.
UFO: Do not forgive Alex's father, do not kill Alex's mother, and save Wheeler.

Collect all photographs to view an extra scene after the credits.

Alternate costumes - Successfully complete the indicated task to unlock the corresponding costume:

Bogeyman: View the "Bogeyman" ending.
Deputy: View the "UFO" ending.
Order Soldier: View the "Drowning" ending.
Orderly: View the "Hospital" ending.
Trucker: View the "Good" ending.

Circular Saw - Successfully complete the game. Then, start a new game session with the cleared saved game file, and search the Shepherd family's garage to find the Circular Saw.

Laser Pistol - Successfully complete the game with the UFO ending. Then, start a new game session with the cleared saved game file, and look on top of the footlocker in Joshua's room to find the Laser Pistol.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Lock 'n' Load (25 points): Found rifle.
Sightseeing (10 points): Found one photo.
Josh's Gallery (50 points): Found all photos.
Alchemilla's Finest (10 points): Defeated one Nurse.
Eddie's Legacy (10 points): Defeated one Feral.
Lurk No More (10 points): Defeated one Lurker.
To The Point (10 points): Defeated one Needler.
Split Personality (10 points): Defeated one Schism.
Shades of James (20 points): Defeated one Siam.
Clear the Air (10 points): Defeated one Smog.
Creeper Reaper (10 points): Defeated one Swarm.
Out of Order (10 points): Defeated one Order member.
Kaufmann's Handiwork (10 points): Found one serum.
Health Junkie (50 points): Found all serums.
The Old Gods... Haven't Left This Place (100 points): Complete game on hard difficulty.
Nursery Rhymes (50 points): Found all the children's drawings.

Additionally, there are sixteen secret achievements:

Six Feet Under (20 points): Defeated Sepulcher.
Blood Donor (20 points): Defeated Scarlet.
Catch Your Breath (20 points): Defeated Asphyxia.
Head Above Water (20 points): Defeated Amnion.
Rising Tension (25 points): Found Circular Saw.
Science Fiction (25 points): Found Laser Pistol.
Smile (50 points): Achieved Ending 1.
In Water (50 points): Achieved Ending 2.
Judgement (50 points): Achieved Ending 3.
Intensive Care (50 points): Achieved Ending 4.
No Dogs Allowed (50 points): Achieved Ending 5.
Mercy (50 points): Alex ended his mother's suffering.
Compassion (50 points): Alex could not end his mother's suffering.
Forgiveness (50 points): Alex Forgives his father.
Angela's Choice (50 points): Alex does not forgive his father.
Now About Those Drinks (25 points): Saved Wheeler.