EDIT: It is possible to miss this achievement by leaving the room mentioned below without listening to Hartman or the tape recorder. Make sure you listen to everything!
As you approach the lighthouse you will eventually re-enter a part of the Cauldron Lake Lodge, where you will find Hartman and a projection of Alan (with a TV for a head) conversing. Listen to their conversation, and then activate the tape recorder when prompted. After the rather...depressing scene is over, exit the room and Alan will begin talking, at which point this achievement will unlock.