Dishonored 2
50 Achievements
40-50h
Heart Whispers
Using the Heart, listen to the secrets of 40 different people
2.5%
How to unlock the Heart Whispers achievement in Dishonored 2 - Definitive Guide
Simply aim at NPCs and use the Heart to learn their secrets. After doing this for 40 different people, the trophy will pop.
You can equip the heart and use it while aiming at people to hear secrets about them. Simply use it on 40 unique people to unlock the achievement. To my knowledge it can't just be used on the same person over and over again, however you can use it on Story characters, guards, civilians, anyone really.
This achievement can be earned at any time during the course of your playthrough.
You will first obtain the Heart during the Void sequence at the start of Mission 2, Edge of the World. In order to listen to the secrets of an individual, all you need to do is equip the Heart and press when aiming the reticle at an NPC. While the achievement description indicates you must do this on forty different people, progress carries over upon death. This means you can make a manual save, listen to the secrets of five NPCs, get yourself deliberately killed, and then repeat this process on the same five individuals eight more times.
If you desire to get this naturally, there are hundreds on NPCs throughout the game's nine missions that you can eavesdrop on. Regardless of how you do it, the achievement will unlock after the fortieth secret has been told.
UPD: for convenience, you can knock him out; this also works with sleeping opponents