To begin eavesdropping, Levantine Assassins would locate their target and sit down on a nearby bench, neither too close or too far from them.
Appearing as nothing more than a civilian, the Assassin would listen in on a short conversation between their target and another person, before using what they had learned to their advantage at a later date.
During the ItalianRenaissance, Assassins would tail Templars or their targets to various locations, before listening to their conversations to gather more information about their enemies. One method they could use to achieve this was to take to the rooftops, enabling them to remain mobile and listen in on conversations at the same time.
Eavesdropping remained much the same to the Renaissance over the centuries, with the only difference being that Haytham Kenway - a Templar raised by an Assassin - and his son Connor had to remain in close physical proximity to hear the target.
In Assassin's Creed, if Altaïr was being chased by guards, they could disrupt his chances of eavesdropping on a target by making them flee. However, Altaïr could eavesdrop once more after he had lost the guards, since his targets would return to the area and resume their conversation.
Once Altaïr had began an eavesdropping mission, regardless of how far he was from the target, he could hear the conversation with equal clarity, as though he were standing right next to the target.