A and C1)
Fighter readies: if the wizard casts a spell I will attack him.
Wizard: 5foot step away and begins casting (without casting defensively).
Fighter: 5foot step and then attack, plus an attack of opportunity due to the casting itself.
Result: squishied wizard with a ruined spell.
There is nothing in RAW to suggest that a readied action prevents the AOO from spellcasting.
Regarding B) No Attack of Opportunity due to the casting defensively. However the Readied action still occurs.
C2: Yes. Immediate actions are immediate and have no bearing on readied actions.
Example:
Wizard steps back using 5foot step
Fighter takes immediate action to step up.
Wizard says swears and reconsiders action. If he tries casting anyhow then the readied action occurs and, if not casting defensively, the AoO.
- Gauss
A fighter is next to a hostile spellcaster and readies an action to "Attack if he tries to cast a spell".
The fighter should specify that he will take a 5' step if needed.
c.1) Will the spellcaster provoke an AoO as well if he does not cast defensively?
Technically, yes.
The caster beginning to cast a spell triggers the readied action.
"The action occurs just before the action that triggers it."
So before he actually casts, the fighter steps forward and hits him. Then he actually begins casting, which provokes, and the fighter now threatens him.
I could easilly see a DM run this the other way, since you only provoke attacks of opportunity when you begin casting a spell, and you must have already begun to cast a spell for the readied action to trigger. But I think the readied action going off before the triggering action takes precedence.
c.2) Would the fighter be able to follow the spellcaster's 5-foot step even if he had moved prior in his round if he had the Step Up feat? (meaning, does taking an immediate action mess up with a readied action?)
It works fine. The caster steps away, the fighter steps up as an immediate action. This doesn't kill his readied action.
Okay, what if the Mage moves a full move, then casts? He'd eat an AOO on the way out, but that would not trigger the readied action, since the fighter can only move 5 feet as part of his readied action. But can't the fighter ready a partial charge which would move him one move?
can't the fighter ready a partial charge
Nope.
Ready: Can you ready an action to charge? (Core Rulebook, pages 198 and 203)
No. The rules for a charge state that you can take a charge action as a standard action if you are "restricted to taking only a standard action on your turn". Although the ready action text states that you can take a standard action, it does not meet the requirements of the text in the charge action.
—Jason Bulmahn, 06/16/11
