Pickpocketing in Assassin's Creed 1?

I've read this problem a lot, but never a solution to my specific problem.

I'm on the PlayStation 3 at Mission where you have to eavesdrop first guy to then steal a letter from him.

I know that you have to pursue your "victim", then get as close to us as possible and steal it with your empty hand. But since it has the same taste as the easy one, Altair always chooses to push him away. What am I doing wrong?

Maybe this video will help you - I did it