There is also a cognitive factor (see ArMDE 07-Hermetic magic): the fast-casting magus has first seen the magus casting the attacking spell, understood, that it is e. g. The Incantation of Lightning and knows, that this specific spell can be blocked by interposing stuff. Had the attacking magus the Minor Hermetic Virtue Exotic Casting (see ArMDE 04-Virtues), this understanding would have been difficult to achieve even for another Hermetic magus.
Against any spell attack, even a magus' shield grog would generally have been completely overtaxed with making such an analysis.
To nitpick- magi typically don’t know what spell is being cast. A check is allowed, but all it does is give them the Form and Technique (and magnitude).
Take a Rego Terran spell- will the magus throw a rock? Teleport the ground from beneath you? Take control of your sword to make you stab yourself? Realistically speaking, the fast casting rules don’t really give enough details for an effective response- Technique + Form + Magnitude still gives a huge amount of possibilities.
In that regard, I don’t think a grog is ‘only’ a step behind a mage, assuming they lack Magic Theory- a grog with magic theory should presumably be able to do the same check.
Quite so. But they have a huge advantage against grogs in the flexibility of their fast-cast defenses:
In order to create a fast-cast defense against magic, a maga must know the Hermetic Form that governs that magic. If the caster is a Hermetic magus and is using words and gestures that she can hear and see, she may determine this automatically. A Perception + Awareness roll may be needed to catch the details of the gestures, particularly if they are subtle.
In other circumstances, the maga must work the Form out. A Perception + Awareness roll against an Ease Factor of 15 minus the magnitude of the effect is needed. Determining Form of Magical Effect: Perception + Awareness vs. 15 – effect magnitude
A grog with little practice in spells and at most minimum Magic Theory needs to identify far more to determine whether she even could block or deflect that spell. Even if by a rare fluke of the dice or a very magnanimous SG she definitely knows that she can (e. g. she correctly identified The Incantation of Lightning and is fast enough), she might risk throwing her life away with that action.
Conclusion: situations with grogs able to block or deflect spells are very rare. Rare enough to not bother with hashing out their real chances in a specific saga with its house rules before the situation occurs and its specifics are known.
I would say realistically, magi should need to know the effect tbh.
It always bugged me following pure RAW, a player might fast cast a creo herbam spell to make a shield against a rock propelling spell- only for it to turn out to be a spell to teleport the ground from beneath the character and for the fast cast to be either completely irrelevant or that the SG has to bend the narrative unnaturally to allow it to apply.
It admittedly always seemed like a bit of fast casting where most tables kinda have to houserule it away to make sense.
A TTRPG is a communication game. One always can play it weirdly, but is not obliged to.
So also Fast-casting defenses should be adjudicated by ST and player jointly to form a working story: defining the result of the fast-cast defense to the effect defined by the ST as it unfolds around the character.
So the player's maga realizes that a Terram magic of the other spellcaster affects stone around her and accordingly controls her spontaneous defense magic - for my sake Creo Herbam. As she notes the stone is moving around her and not towards her, she e. g. slows it down by padding, covering or diverting it with wood, depending on how it moves ... and so on. If the stone moves away she might thus end on a wooden platform.
The point is to make a paragraph of rules work in a the game. And if the ST is not able to do so - but best not before - the troupe applies and records house rules.
There's nothing really implying that, and ignoring it what you bring up is a table resolution-
Rules as written all the player gets is the Form, Technique and magnitude of the effect. The SG might be willing to divilge more information, but thats deviating from the core of the problem: fast cast defenses don't have enough context.
I assume it is more of a behind the scenes magical result the interrupting mage doesn't have to know the exact spell but instead uses a FoTe to disrupt the spell as it is cast. If successful it manifests as some form of appropriate block or defense but the defending mage doesn't consciously determine what that is.
Sorry- was rereading some forums and restumbled upon this.
I don't think that really makes sense. If a magus forms a shield to block a rock propelling spell, but in turns out the spell is to create teleport the ground beneath them- well, how does the shield help at all? I can't imagine it helping against preventing falling or nullifying fall damage, unless you added a Rego requisite as you were casting the spell.
Hence why it always bugged me- Tech+Form+Mag is a massive range of possibilities and just those are not able to get you a certain spell