Here is the full list of all 20 The Guest achievements.
Get to the end.
Write BENEFACTOR on the typing machine.
Try lighting a cigarette.
Call every single number on the phonebook.
Use the sign and don't be disturbed.
Locate the Team Gotham's headquarter.
Type the password in the computer.
Get into the secret room.
Turn off all the lights.
Listen to some music on the record player.
Try consuming the bath pills again.
Collect all possible objects.
Complete the dream maze scene withouth failing.
Read all the newspapers.
Complete the music puzzle without failing.
Enter the toolbox password without combining the notes.
Wait 8 hours and hit the button again.
Complete the valves puzzle in less than 20 movements.
Be thankful twice.
Solve the batteries puzzle in just 2 movements.