Does this require a feat? If not, does itself provoke an AoO?
For example, my cleric is standing between some guards and the rogue picking a lock. One of the guards walks through the square next to mine. In place of my AoO, can I attempt to trip him?
Best Answer
Yes, you can, if the Combat Maneuver is made instead of a regular attack.
Look at the descriptions of Combat Maneuvers. Some of them state specific actions, such as Bull Rush, where it says:
In this case, you can not substitute your AoO for them, as you don't actually get a Standard action (or any other action type). This is for the same reason that you can't cast a spell as an AoO.
Others, specifically Trip, Disarm and Sunder state:
(or equivalent phrasings). You can use these three as part of an AoO.
Regarding provoking an AoO
Note that even if you attempt a combat maneuver during an AoO, you are not exempt from the usual rules of combat maneuvers provoking an AoO. You can prevent them by having the appropriate Improved ... feat (such as Improved Trip).
Your example