The original question was re: Images and AoOs, so for that situation all in one place is a good way to rule it rule-wise. Actual game/display-wise, I spread em out as appropriate
Doing so negates the players ability to use images tactically, which is why I mentioned that it was merely a suggestion, not actually RAW. RAW, the fighter would be able to make an Aoo in the illusion, at which point he would interact with it and realize it to be false, but still use up one of his aoo's. However, unless he had a reach weapon, or reach greater than 5', he should not have had any chance at all to stop the wizards spell, and using the ruling mentioned would give him such a chance.
Situation: In combat.
Character A is successfully hidden. On his turn, character B readies an action "Charge and attack any enemy beginning to cast a spell.
Character A, still hidden, begins to cast a spell.
How do these two actions interact ? Does character B get his readied action ? Must he succeed a spot check (opposed to character A's hide roll) - if yes, the original hide roll ? A new one - or one at a penalty ?
2) Can a hidden character use the "sniping" option to cast a spell and hide again (at the -20 penalty) ?
I'd say that casting a spell is functionally no different than making any other form of attack. If Character B manages to notice Character A in time (i.e., beats A's Hide Check with his Spot Check) he can use the declared Readied action. Generally, a new Hide Check is only required after you do something to attract notice.
Can a caster snipe? Why not? There isn't much mechanical advantage to be had in doing so, but if a player wants to invest the skill ranks to make it worthwhile I say let them.