How to Unlock Curiosity Killed the Cat Achievement
Can only be done in “A Morbid Dance in the Second Line” (scenario 8). While passing through the level you will come by Lanuit’s shop again.
This time he is not there to stop you from investigating any of the items on display. Move to the back of the store where a skull is glowing in purple light and examine (the second option) it. You don’t have to actually take it with you to unlock the achievement.