This page of our guide to Sherlock Holmes Chapter One describes the minigame, during which you have to eavesdrop on unaware characters. The first such minigame will appear in Ghost of the Past - seance room. Every eavesdropping works in a similar way. This page describes the mechanics of this part of the gameplay, and the walkthrough contains the exact instructions for each mechanic separately.
You can only eavesdrop on people who are marked with a special icon. It's an ear icon, which you can see on the screenshot above. Such a task may appear during a story mission; you are unlikely to come across this icon when free roaming through the open world of the game.
Eavesdropping involves selecting the most important information from the entire conversation. You always have to think about what is relevant to the case. Only these pieces of information must be approved to remain on the screen.
Any information that is not relevant to the case should be rejected. Simply click in them with the right mouse button to make them disappear and not interfere with solving the case.
Every eavesdropping has its limited time. You need to select the correct information quickly. When eavesdropping, you must collect all information about the case - you can't miss any.
Remember that bad decisions will shorten the time limit and eventually may stop the minigame altogether.
You can repeat the minigame many times, though, to eventually find the solution.