While on the streets of Dunwall (after escaping the palace) in Mission 1: A Long Day In Dunwall, you will pass by a place called "Dunwall Courier". It is a building on the left of the road before going to the harbor. Go to the top floor to see the Printer being threatened by a soldier. Stab the soldier, then talk to the Printer afterwards (he will run into a side room) to get the "Freedom Of Speech" achievement after the dialogue.