"Assassin's Creed" doesn't confine you to a series of assassinations, as you can also pickpocket enemies and neutral targets. Pickpocketing civilians can get you into trouble with the authorities if you're caught, but pickpocketing missions also become available over time. On these missions you're able to pickpocket a specific target for a specific goal. Holding down the correct button as you bump into your target will let you rifle through his bag and steal what you're looking for.
Find the target of the pickpocketing mission on the map and go to that location. Stand close enough to the target to hear his conversation but not close enough so that you'll be noticed. This may take a few tries to get the distance exactly right. Lock onto the target to begin listening to his conversation.
Follow the target after the conversation has ended. On most pickpocketing missions, the target will walk away at a normal pace, so match his speed and approach him. Targets on pickpocketing missions are on their guard, so don't rush them or do anything hostile to make them run away.
Hold down "B" for Xbox 360, "Circle" for the PlayStation 3 or "Shift" for the PC as you bump into the target to pickpocket him. Do this when the target is looking straight ahead or you may be spotted and lose the mission. If you're successful, the target will turn around and check his pouch for something missing, but he won't suspect your involvement.