The Hidden Blade Assassin's Creed in Depth

Step by step instructions on how to assassinate Robert de Sable very quickly:

1. Before the assassination go into Options, Controls and turn on the "Hold Trigger to Lock" option.
2. Step into the circle of Templars and get in Robert's face.
3. When Robert says "To arms, assassin.", push the Left Thumbstick northwest.
4. Upon the first white flash, press D-pad up to select the Hidden Blade.
5. Wait about 1/4 second before pressing the attack button.

You only get one shot at this per Memory Block playthrough. If you die and reload, Robert is already safely behind the blue memory wall.