imaloony8.0

You can't actually pull it out so that you walk around with it ready, but you can use your knife for certain actions such as prying a vent open or breaking a lock. Also, if you run up behind or in front of and enemy and hit the right trigger you'll attack them with your knife and kill them (Left trigger knocks them out). Also, if you sneak up behind an enemy and hit A when the hand sign appears, you'll grab them and hold your knife to their throat. From there, you can interrogate them with A (If they have anything interesting to say), Knock them out with Left trigger, kill them with the right trigger, or drag them around.

Hope that helps.