SlimGauge wrote:The closest I think you can come is to ready an action to AoE the general vicinity that you hear verbal spell components come from. This doesn't "counter" the spell, but you might disrupt casting by causing damage and a concentration check.
While this might "seem" a legit option, I see several issues here.
1-. You cannot target someone as the effect of a readied action unless you have LoS. That effectively prevents "Distract" and "Counter" readied actions and limits you to area-based readied actions.
Readying has little to do with it; you can't target someone you can't see. But you can certainly fireball the general vicinity where you hear someone speaking a verbal component.
2-. Talking about area-based actions, if you ready an "I cast a spell in response of another spell being cast somewhere around here" action... How do you know a spell is being cast unless you roll a spellcraft check? You might be better with "...I hear sound", but that can lead to other consequences.
Spellcasting doesn't sound like ordinary speech. While an invisible caster could try to trick you into shooting early by Bluffing you with nonsense phrases, he needs special powers to conceal verbal components as something else. Since we're not doing conventional counterspelling we don't care to much about identifying the exact spell.
3-. Talking about sound, readying an action that way doesn't automatically make you become a 360º sonar. You'll still have to roll an oppossed Perception vs stealth check (and invisible creatures have hefty bonuses to stealth).
Casting requires you to speak in a strong voice (which is an easy DC), and I don't think Invisibility bonuses to Stealth are supposed to apply to audio-based Perception checks. After all, Invisibility isn't preventing you from speaking in a strong voice (that would impair your spellcasting like a Silence spell does).
4-. Finally, even if you succeed on the oppossed check or recognize a spell being cast... if the target casts a spell with a cast time of a standard action and you cast a spell with a cast time of a standard action... the target's casting must be already on it's way before you recognize the gibberish and start casting yours. That would lead to a hefty spell economy and casting time discussion. Per RAW you should cast first... but if your target casts fireball and you take half a second to recognize something is being cast and you decide to cast fireball... I'd totally say your target's fireball will pop 1st.
The rules for readied actions are clear: the readied action goes off first. Even though 95% of all readied actions take just as long as the things they're readied against. Them's the rules of the game.