Step-up and strike + guy moved and tried to cast a spell

[cmastah]

I know this has been asked before but I can't find the thread.

Let's say that a guy wanted to take a 5-foot step and then cast a spell, right? When would the other player (who has step-up and strike, who we'll call the stalker for now) take the 5-foot step and the AoO? If the stalker immediately followed up, the caster would probably NOT cast his spell (or do any other range of actions, like maybe use a quickened spell instead), so could the stalker decide when during the caster's turn that he chases after him? For instance the stalker waits until the caster is about to start casting the spell THEN chooses to follow him and do an AoO?

[mdt]

Step Up wrote:

Step Up (Combat)
You can close the distance when a foe tries to move away.
Prerequisite: Base attack bonus +1.
Benefit: Whenever an adjacent foe attempts to take a 5-foot step away from you, you may also make a 5-foot step as an immediate action so long as you end up adjacent to the foe that triggered this ability. If you take this step, you cannot take a 5-foot step during your next turn. If you take an action to move during your next turn, subtract 5 feet from your total movement.

See the bolded section, you may take an immediate action whenever an adjacent foe attempts to take a 5 foot step. There is no wait and see what he does. You either take the immediate action when it's triggered (the 5-foot step attempt) or you don't. It's really just as simple as that.

[Darksol the Painbringer]

MDT pretty much covered it.

RAW, The proxy for the Step Up (and it's correlatory feats) is listed as when the target makes the 5-foot step activity, and they must move a distance away from you (not sidestep or move adjacent to you). When this proxy occurs, the creature can then choose to follow it, or not. If not, then the proxy is ignored.

It's no different than triggering an Attack of Opportunity. The creature can choose to ignore the opportunity to attack (and save their Attacks of Opportunity for another target), make an attack of opportunity at the right time of provocation, or even make it as soon as it triggers. However, once the creature ignores the option to make the attack (for whatever reason), it can't just wait for the player to make their decision (and decide to expend attacks of opportunity based upon the decision they make after the proxy), its chance already occurred, and it was ignored/wasted.

Allowing it otherwise would be considered meta-gaming and/or retconning.

[cmastah]

Thanks guys :)