Grayhawk said:
3: You can try to trip as the AoO you get from someone trying to stand up, and a successful trip attack knocks the person back down, making him lose his MEA.
Bonus question: If you have Improved Trip do you get to make a free attack if you successful trip on an AoO?
Bonus question 2: If a caster is tripped during casting, does he still get to make a concentration check to get the spell off?
Option #3 would be correct. If you attack a Wizard when he casts a spell, he has to make a concentration check to finish the casting. He doesn't get to "try again" at casting the spell without losing his action.
Thus, if you provoke an attack of opportunity by trying to stand, and are tripped again, your move action is wasted, and you're back on the ground.
If a prone target provokes an attack of opportunity by doing ANYTHING, even trying to stand up, the opponent should get the +4 to attack on him. He hasn't actually stood up yet, he's only TRYING.
Yes, if you make your AoO a trip attempt, and sucessfully trip, you get a free attack per Improved Trip.
If a caster is tripped by an AoO he doesn't have to make a concentration check at ALL. He hasn't taken any damage, and to my knowledge, there is no penalty from casting while prone. If he has to make a touch or a ranged touch attack, he may suffer penalties from this. If the person tripping the caster had Improved Trip, and hit the caster with his free attack after tripping, then the caster WOULD have to make a concentration check against that damage.
I think I'm right on all this, but others can weigh in on it..
