Artoomis said:
Off topic (sorry, but it needs to be done):
The Attack of Opportunity will take place just before he attempt to stand up. You can't trip him, because he's not up yet. He still gets to stand, but he takes an AoO as well. He can also still take a Standard Action - if he moves AND if the opponent has Combat Reflexes THEN he might be tripped again with the second AoO.
People keep saying this, apparantly with confidence, despite it being unsupported by both the core rules and commen sense.
The AOO is RESOLVED before the action that triggered it. This is not the same as saying the AOO occurrs "before" the action that triggered it (in the flow of game time). Because that makes no sense.
"I attempt to cast a spell."
"The enemy fighter hits you with an AOO"
The spell is in the PROCESS of being cast, the AOO is resolved, and based on that something happenes (the spell fails, it goes off, whatever).
Obviously if the spell is interrupted, the spell is lost; the AOO does not happen BEFORE the spell is started, but it is RESOLVED before. IF the AOO happens before the spell is started (in game terms), no spell could ever be interrupted by an AOO.
Similarly if somebody is standing up from prone, and provokes an AOO, the attack is resolved before the action is completed, but the action is "started" i.e. somebody is trying to stand up. How far up is he when the AOO goes off? The game does not say.
Now, D&D is turn based; it is not simultaneous; AOO's have to work this way in this kind of system. It's also not precise; you are left to fill in the details. Joe attempts to stand up, and Mary gets an AOO, chooses to trip, and trips him.
According to the rules, Joe could have been 1% finished standing up, or 99% finished; a spellcaster could be 1% started casting a spell, or 99% completed when the AOO goes off. It's not specified, and that much detail is not needed.
You claim you can't "trip" somebody who is attempting to stand up. What makes you think this? The person you are tripping could be 99% standing up by the time the AOO goes off (completely standing except for one bent knee) or 1% up (just supported on one hand/elbow). Why would your disallow somebody that's 99% of the way up to be tripped? Why do you think it's impossible to "trip" sombody prone that's trying to get up, even if they are still prone? (meaning put them competely down again)?
And more to the point, the core rules don't state you can't trip a prone creature. (to my knowledge).
In my view, Joe blew his move-equivalent action attempting to stand up; Mary tripped him back down. Joe lost the action. Now Joe can try again, provking another AOO from Mary if Mary has Combat Reflexes; if not, he stands up. If Mary has CR, he might get tripped down again.