Tickle William's face with the feather — not his feet — until he says stop.
Official in-game clue: "You have a feather! Toes aren't your only victim."
Where: Day 2 Morning, during Waking up William.
Why it gets missed
Feet are what you tickle someone with a feather, and the game lets you do exactly that. It works as a way to wake him, so the scene advances and the achievement does not fire.
The clue names the trap outright: toes aren't your only victim. It is telling you the interaction you already found is not the one being graded.
What to do
- Take the feather.
- Use it on William's face.
- Keep going until he asks you to stop.
Waking William has a second layer
Separately from the tickling, there is a guess-and-read loop: click his hat, ears and tie, and the direction his eyes point tells you which to try next. That is its own interaction and worth doing while you are here.
Day 2's other missable pair
Roasting the coffee is only half the job — grind it too, and the brush belongs on the kitchen portrait rather than the floor.
Nearby
The full list of hidden achievements, in the order you reach them, is on the triggers page. Going for everything in one run? See 100% completion.