Do creatures subject to Stand Still or Pin Down lose the action that provoked the feat?

Question

Pin Down

Benefit: Whenever an opponent you threaten takes a 5-foot step or uses the withdraw action, that opponent provokes an attack of opportunity from you. If the attack hits, you deal no damage, but the targeted creature is prevented from making the move action that granted a 5-foot step or the withdraw action and does not move.

Stand Still

Benefit: When a foe provokes an attack of opportunity due to moving through your adjacent squares, you can make a combat maneuver check as your attack of opportunity. If successful, the enemy cannot move for the rest of his turn. An enemy can still take the rest of his action, but cannot move. This feat also applies to any creature that attempts to move from a square that is adjacent to you if such movement provokes an attack of opportunity.

If a creature is subject to one of the above feats, does it enact the remainder its turn as if it never did the action that provoked the feat, or is it treated as having done the action? For instance:

1. If a character attempts a 5-foot step and is stopped by Pin Down, does it count has having 5-foot stepped (albeit ineffectually) and therefor cannot move for the remainder of its turn, or can it still take one or more actions to move?

2. If a character adjacent to someone with Stand Still attempts a move action and is stopped by Stand Still, can it still take a full-round action that does not involve movement, or only a standard action?

3. If a character can take a full-round action after being stopped by Stand Still, what if the character started a move action and was stopped by Stand Still after moving 5 feet? Does that count as having made a move action or a 5-foot step?

Answer 1

This is a case where the rules are a bit unclear, particularly in the feats, and I think different people will interpret these questions differently. It's worth looking at the rules text for timing on attacks of opportunity:

An attack of opportunity “interrupts” the normal flow of actions in the round. If an attack of opportunity is provoked, immediately resolve the attack of opportunity, then continue with the next character’s turn (or complete the current turn, if the attack of opportunity was provoked in the midst of a character’s turn).

So essentially, this is how the actions would flow:

• Foe uses the "Move" move action and begins moving out of a square you threaten.
• Your AoO immediately procs, and is resolved now.
• If the foe is still able to use their action, they may continue, however if your AoO disrupts the triggering action in such a way that it cannot continue, that action ends and is used up.

This order of operations with AoO's comes up a lot when standing from prone (Are you still considered prone when provoking an AoO while standing? Yes, because the AoO resolves before the triggering action finishes resolving)

So to answer your questions:

1. Yes, they began using their 5ft-step/withdraw and it was disrupted. The action they spent to use that action is already spent. If this wasn't the case and Pin Down effectively "rewound" the action order, they could just attempt to 5ft-step over and over again until you ran out of AoO's!
2. No, the creature has already spent some amount of action (usually a move action) to attempt to move, and Stand Still disrupts it. However it may spend any other actions it has remaining, so long as they don't cause that creature to move from the space they're pinned to, such as taking another move action (though the text is a bit ambiguous on this). This is in contrast to normal AoO rules, where taking an AoO does NOT disrupt the movement (unless it placed the target in a state in which they could not continue, such as being tripped or knocked unconscious).
3. We've already answered the question about Stand Still, but taking the Move action (by spending a move action) is an entirely separate action from taking the 5ft-step action for free.