Sune Nødskou has creasted a Terror Deck, a play-aid for Trail of Cthulhu based on the ideas in the ‘Roleplaying Instability’ sidebar on page 72 of the core rulebook.
The deck consists of 60 card divided into three separate decks, called Chill, Fear, Dread and Terror. Each deck symbolizes a level of fear taken from the text box. (Chill: loss of 1-2 Stability points, Fear: loss of 3-4 Stability points, Dread: loss of 5-6 Stability points and Terror: loss of 7+ Stability points.)
Each card then have a suitable, erratic, unstable behavior written as a suggestion and keepsake to the player. The levels of irrational behavior is meant to reflect the four “levels” from the ToC rulebook.
Instead of asking the player to invent a strange behavior, ask the to pick a card from the right category, and have let them keep it face up in front of them, until the insanity is over. These can be printed out and stuck onto a set of standard playing cards.
You can download the PDF here.