Nail said:Nonsense.
Nonsense.

Nail said:#1) How do you know "what has already taken place?" => By the DM telling you.
#2) How does the DM "tell you"? By describing what happens.
#3) Can you interupt your DM before he's finished describing what happens? => Absolutely.
The issue really becomes: How rude do you want your players to be?![]()
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From a mechanics point of view (i.e. allowing the players to get in immediate actions that they should be allowed to do), your point here is really non-sequitor to the discussion.
The DM should announce intent before he rolls the dice and he should only announce information that the players should have.
If he is going to be a jerk and not do this or do this in a manner that he relays information to the players that they should not yet have, that's his problem (and a problem for his players).
The DM should announce: "The NPC Wizard is casting a spell".
He could also announce: "Give me a Spot check. 23? You notice he is looking right at you."
He could also announce: "Give me a Spellcraft check. 12? 15? 26? It is a Fireball spell, but only Frodo knows this because he is the only one who made the roll."
All of these give the players a chance to react by casting an Immediate spell.
The DM should not pick up his dice, roll them and announce: "The NPC Wizard is casting a Fireball spell at Lego, Frodo, and Dimwhit and hits them for 40 points of damage.".
This could create problems when the player announces he wants to cast an immediate spell: a) because the player has more info than he should, and b) the DM is rushing combat to the point that reasonable actions by the players seem like metagaming.
If he does announce like this, then it is not rude to tell the DM that you have an Immediate spell that could affect the situation and that you would appreciate it if he would not just blurt info out like this and instead, give the players a chance to make decisions while the Wizard casts.