I Have Seen People Have Problems Unlocking This One, And I Couldn't Unlock It Just By Switching Between All Ten Ghosts.
- What I Did To Unlock It -
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On The Terrortown Level -
Go All The Way Left, To The Museum, And Start Scaring People.
- Once Your Atmosphere Goes High Enough To Unlock Another Character, Do So!
(I Unlocked The Pharaoh First)
- After You Unlock A Character, Immediately Stop Scaring People, And Go Switch To Them.
The Order I Unlocked Them Was: Pharaoh, Thief, Nurse, Clown, Phantom Of The Opera, Jester, Cook, Diver, And Finally Captain.
Switching To The Captain Netted Me The Achievement.
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- I Did Not Use Any Ghost Emotes (Boos)
- Switching Between All Ten Ghosts Towards The End Of The Level Did Not Seem To Work, But Unlocking A Character With Each One Did The Trick.
(Reports Of People Able To Just Change Into All Ten Characters For The Achievement Are Sporadic)
Good Luck!