Here is the full list of all 27 The Beast Inside achievements. It takes around 10-12 hours to unlock all of the achievements on Windows.
Complete Chapter 1
Find the Key to the Attic
Choice No. 2 (Evil Nicolas)
Find all hidden messages
Shoot down all anomalies
Collect 10 notes as Adam
Collect 50 boxes of matches
Light 30 candles
Collect 10 notes as Nicolas
Collect 50 bottles of kerosene
Ending No. 3 (Adam kills Emma)
Ending No. 4 (Adam kills himself)
Solve the Enigma
Unlock all achievements
Collect all notes as Adam
Collect all notes as Nicolas
Ending No. 1 (Adam is sick)
Collect all newspaper clippings
Chase away Norton's Ghost 10 times in the woods
Finish the game
Defeat John Norton
Ending No. 2 (Adam kills the doctor)
Hear every news message on the radio
Find 10 hidden messages
Choice No. 1 (Good Nicolas)
Defeat Walter Gilman
Find all Harold's notes