Arkhandus
First Post
AoO's come at the same time as the action that provokes them. They do not interrupt or prevent the action, unless specifically stated (i.e. the Stand Still psionic feat specifically says that it interrupts the foe's movement and prevents it if the AoO and opposed Strength check succeed). A spell being cast is simply interrupted by a successful AoO because it disrupts the chanting and gestures - think about it, if some guy just slammed a quarterstaff across your jaw, would you be able to keep your hands steady and your mouth still chanting coherantly at that exact moment? No, your speech will either be paused or interrupted by a yelp or grunt of pain, and your hands will likely clench reflexively from the blow, disrupting the intricate spell gesture sequence. That is why spells are interrupted by an AoO that successfully damages you, the AoO itself doesn't stop it.
Likewise, if you're tripped by an AoO that was provoked by you standing up from a prone position, you were already halfway standing when opponent noticed and swung their weapon during your moment of vulnerability. If the trip succeeds, you end up on your backside again, but your standing motion is only interrupted because you now have to start over at getting up, in which case you might take another move-equivalent action to do so, but your current move-equivalent action is already more than half-expended. If the AoO had been just a regular attack rather than specifically a trip attempt, you would have still stood up fully afterwards with the same action, you just would've suffered a blow as you were getting up, but not necessarily an unbalancing one that would knock you back down like a trip. The opponent shouldn't get a +4 for attacking you prone, because you were already starting to get up and regain your mobility, at least in my opinion.
Also, note that any given creature can only take 1 attack of opportunity per round against any particular foe. If you try to stand once and are tripped in the process, your second attempt that round will not provoke an AoO from the same opponent who just made an AoO against you.
Likewise, if you're tripped by an AoO that was provoked by you standing up from a prone position, you were already halfway standing when opponent noticed and swung their weapon during your moment of vulnerability. If the trip succeeds, you end up on your backside again, but your standing motion is only interrupted because you now have to start over at getting up, in which case you might take another move-equivalent action to do so, but your current move-equivalent action is already more than half-expended. If the AoO had been just a regular attack rather than specifically a trip attempt, you would have still stood up fully afterwards with the same action, you just would've suffered a blow as you were getting up, but not necessarily an unbalancing one that would knock you back down like a trip. The opponent shouldn't get a +4 for attacking you prone, because you were already starting to get up and regain your mobility, at least in my opinion.
Also, note that any given creature can only take 1 attack of opportunity per round against any particular foe. If you try to stand once and are tripped in the process, your second attempt that round will not provoke an AoO from the same opponent who just made an AoO against you.